« PreviousContinue »
son the work will be of special interest to the plant physiologist. The abstracts used in the preparation of the several monographs have been accurately prepared and the numerous citations to the original literature enhance the value of the work very much. If in courses of plant physiology it were required of the student to isolate all of these plant products it would add very much to the student's competence to deal with the nature of physiological processes and make this subject of greater fundamental value in its application to other phases of botanical work.
The second edition has been improved by re-writing the section dealing with the chemistry of plant pigments. This subject during This subject during the past few years has been the field of most interesting study by Willstätter and Miss Wheldale.
The value of the work would be much enhanced by a somewhat different arrangement of the subjects, particularly if they could be connected in their biological relations. To begin with "fats, oils and waxes," substances which are seldom studied in courses in either plant morphology or plant physiology, tend to discourage the use of the book by those for whom it would be a source of greatest benefit. On the other hand if a subject like the carbohydrates or pigments were first considered, both of which are under constant observation by the student of botany, it is quite likely that the chemical methods contained in the work would be applied in laboratory instructions. Furthermore the microchemistry of all of the plant products considered should be considerably improved upon. Many of the statements are only partly true; in others more pronounced reactions could be utilized while in still others a large amount of work should be included. To-day the interest in microchemistry or chemical microscopy is very great and with the appearance of such excellent works as those of Chamot, Molisch and Tunnmann there is an excellent basis in a work of this kind to connect the morphology of plants with the chemistry of the constituent and to follow physiological processes with microchemical reactions. HENRY KRAEMER
June 1, 1918
SCALARIFORM PITTING A PRIMITIVE FEATURE IN ANGIOSPERMOUS SECONDARY WOOD
PROFESSOR JEFFREY, in his recent stimulating book, "The Anatomy of Woody Plants," Ch. VII., derives the vessel with the simple or porous type of perforation from the fusion of horizontal rows of circular pits in the endwall, the scalariform pit and perforation being merely an intermediate stage in the process. Perhaps such a reversible evolution has gone on in certain groups, although it is here attempted to show that the available evidence is capable of the opposite interpretation.
Multiperforate and even uniperforate endwalls in gymnospermous vessels may arise from the fusion of circular pits with the dissolution of the closing membrane as Professor Jeffrey describes for Ephedra; but this fact seems inadequate to explain either the presence of scalariform pitting or the wide prevalence of scalariform perforations in the vascular elements of the less specialized angiosperms. It does not appear that all of the facts germane to the subject have been fully considered. In tracing out the development of vascular elements with uniperforated end-walls in accordance with Jeffrey's hypothesis, serious difficulty is met.
The scalariform pits in conservative regions of the secondary wood of Liriodendron, Drimys, Asimina, and other forms with prevailing circular pits in the less conservative regions, suggest antecedence of the scalariform condition, while a further illustration is afforded by the monocotyledonous Dracana. Thus, in Dracæna aurea, typical secondary xylem without vessels is formed, the fibro-tracheids of which have circular pits in their lateral walls, but typical scalariform pits in the walls of the overlapping tracheid ends. There is no indication that such scalariform pits have arisen from the fusion of rows of circular pits. They are evidently a primitive feature of the tracheid and closely resemble the scalariform pits of the secondary wood tracheids of Drimys found in the vicinity of the pith; yet only a slight modification of the tracheids, with dissolution of the closing membranes and borders of the scalariform pits, would com
plete transformation into typically perforated of the bars in the perforation may become scalariform vessels. more or less intricate.
It is evident that, to the extent that adjacent cells become specialized, and unlike in shape, in size, and in function, as, for example, the tracheal segment and an adjacent prosenchyma cell of the angiosperms, the scalariform pit must lose its alignment as an intercommunicating structure. Conversely, the circular pit is the more adaptable, and prevails in the vascular elements of more advanced plant families as typified by the Compositæ. Just as might be hypothesized, the scalariform pit is relatively more common in the vascular elements of less specialized families included in the Ranales. Probably the scalariform pit prevailed in the early angiosperms, and is even now being slowly discarded for the smaller circular pit. It was, in case of the vessels, first discarded on the lateral sides adjacent specializing tracheids, ray-cells, wood-parenchyma, or, especially, fibro-tracheids and fibers. The close relation between a vessel and the adjacent element or elements is evident from the fact that, for example, in Cheirodendron (Araliaceae) four distinct types of pits communicate respectively with prosenchyma, wood-parenchyma, ray-parenchyma, and with other vessels. However, that the end-walls should preserve the more primitive sculpture is quite in harmony with the fact that adjacent cells are, in this case, alike. Complex modification is here unnecessary, and adequate comparisons of the secondary wood of existing primitive and specialized families proves the correctness of the view advanced. It is indeed a remarkable fact that in woods with scalariform perforations in the vessels, the prosenchyma usually bears distinctly bordered pits and is thus less distinct from the tracheal segment than in case of woods in which the vessels are characterized by the simple or porous perforations.
Comparison of such types with the scalariform wood of Cycadeoidea Dartoni has been suggested by Dr. Wieland, and for the purpose he has placed before me recently cut sections. The preservation of this fossil is perfect. The sections show the minutest detail in the pitting of the tracheids, even under a magnification of 450 diameters. The outline of the pits, the pit-apertures, and other minute characters are preserved in every detail. The sides and ends of these tracheids, in both tangential and radial aspects, are pitted with regular scalariform bordered pits, which at once remind one of those of Magnolia. Along with the true scalariform pits occur a few elliptical pits, and these are inserted between the long pits in such a manner that, by the dissolution of the borders and closing membranes, occasional branched bars would result. The resemblance between the pitting of these cycadeoidean tracheids and the vascular elements of Magnolia on the one hand, and of Dracaena on the other, is perfect, and may be followed out in minute detail.
A feature of interest not mentioned by Jeffrey is the more or less frequent occurrence of branched bars. They occur occasionally in Liriodendron and other genera with scalariform perforations; but in Cheirodendron and some other araliaceous woods this branching
The evidence in support of the hypothesis that scalariform pitting is primitive is convincing. It is evident that the process of the breaking up of scalariform pits into circular pits was in progress in the antecedent cycads, and that this process started first in the tracheid side-walls, whereby the overlapping tracheid ends became more conservative than their truly lateral portions. The complementary relations in the wood of the cycadeoids and cycads outlined in Wieland's recent note1 are thus anatomically reconciled. The origin of branched bars is also explained.
Excellent examples to show that perfect scalariform tracheids exist in living types occur in Magnolia hypoleuca. The scalariform pitting very closely approximates that of Cycadeoidea Dartoni. Scalariform tracheids slightly more advanced may be found in the aquifoliaceous Byronia sandwicensis Endl. Here the tracheids retain typical sca
1 Wieland, G. R., Feb., 1918, "Cycadoid Wood Structure," Science, N. S., XLVII., pp. 141, 142.
lariform bordered pits at the overlapping ends, with the exception that occasional shorter pits are present and in a position to form branched bars should the pit membranes and pit borders be eliminated. The lateral walls show perfect transition from scalariform to circular pits. Living types, therefore, preserve all stages in the transformation of scalariform tracheids into vessels with multiseriate circular pits and simple perforations; and there is every reason to believe that modification is still going on.
Many seem to be under the impression that scalariform pitting is of rare occurrence above the cycads. A close comparison of scalariform tracheids, which Wieland's material makes possible, can leave no doubt that existing forms, in dicotyledons as well as in monocotyledons, still exhibit, in the vascular elements of their secondary wood, almost complete stages in the transformation of scalariform pitting into that of the circular multiseriate type, affording a valuable criterion by which to judge the relative primitiveness of angiosperm groups. The histologic evidence is fairly in accord with the floral evidence.
The exceptional abundance of circular pits in such forms as Vaccinium corymbosum, noted by Jeffrey, is accentuated by the fact that, in this type, the vessels are mostly isolated from one another and in contact with wood-prosenchyma which forms circular bordered pits in common with the vessels. Scalariform pitting occurs near the pith where, occasionally, vessels are adjacent. Here occur vessels showing perfectly the transition from scalariform pits to scalariform perforations, as well as the transition stages, noted by Thompson, from scalariform to simple or porus perforations. FOREST B. H. BROWN OSBORN BOTANICAL LABORATORY, YALE UNIVERSITY
THE ORIGIN OF DICOTYLS
IT is generally conceded that the origin of the conifers is from the evolutionary point of
2 Thompson, W. P., Jan., 1918, "Independent Evolution of Vessels in Gnetales and Angiosperms, " Bot. Gaz., LXV., pp. 89-90.
view a fairly luminous subject. Neither stem, leaf nor cone has wholly new or remote features. The group appears rather old mainly because so much of its history is known or directly inferable. Evidently, sporophyll consolidation into the hard spiny unisexual cones mainly occurred in the Permian, and by Jurassic time was complete. There is, thus, the distinct connection with the dominant Cordaites of the ancient world, and the cycads and Ginkgo amongst still existent types. Moreover, that Ginkgo was cosmopolitan, and of varied form in the Rhätic further broadens the possibilities of conifer relationship with known types, while by way of seed-ferns the several gymnosperm lines are in common carried back to Pteridophytes.
Not so the origin of dicotyls, long wholly without the range of scientific discussion. It was understood in a general way that the monocotyls did not obscure the problem. That they were either a contemporaneous or later development appeared more or less certain. Before the discovery of the cycadeoids, however, no line of attack from the viewpoint of either fossil or existent evidence seemed to carry the dicotyls beyond the Cretaceous. Back of that period the decipherable record ceased entirely. But the discovery that the cycadeoids were a varied group with flowers recalling those of magnolias, at once suggested that the origin of the dicotyls could not be wholly obscure. It was soon seen that the great extension of Magnoliaceous species in the lower Cretaceous must indicate a series of more primitive magnolias in the Jurassic. There might have been doubt as to the significance of the cycadeoid flower with its pollen-bearing synangium. But the reduced sorus of existing cycads plainly stood in the complementary relationship, just as did the cycadeoid microsporophyll and the carpellary leaf, as components of cycad fructification.
Nextly it was found by Nathorst and Wieland that the larger proportion of Mesozoic cycadales were cycadeoids, in part bearing small and reduced types of flowers. The Wielandiella of Nathorst with its slender bifur
cating stems and Nilssonioid foliage proved the form of greatest interest; although very recently another related form, Williamsoniella, of quite equal interest from the Yorkshire coast has been reconstituted by Thomas. This has a very small perfect flower with somewhat scale-like microsporophylls, bearing, as it turns out, only a few pairs of sessile synangia. The stems are slender, and the foliage definitely Nilssonian to Tæniopterid This relationship was in Wielandiella somewhat obscured by a peculiar prolongation of the midrib.
Now this definite inclusion of Nilssonians amongst the small-flowered cycadeoids demonstrates the presence of a great alliance related to the primitive magnolias and abundantly represented in the leafy records of the Mesozoic. For it is seen that to this group must be added not only Taniopteris but Macrotaniopteris at least in part. In 1905 I called attention to these foliar types in Monograph XLVIII. of the U. S. Geol. Sur. in the following justifiable terms:
The genera Nilssonia, Taniopteris and Oleandridium have now come to comprise numerous species of a very generalized and cosmopolitan type of leaf. As a consequence, it has become difficult, as always in such a case, to say definitely, in the absence of extended revision, where the one genus ends and the other begins. Nevertheless we have every reason to believe that at the one end of the series there are characteristic ferns analogous to such living forms as Oleandra and Acrostichum, as well as marattiaceous forms, and at the other an important list of cycadaceous forms. The closely related genera Pterophyllum and Anomozamites [now Wielandiella, in part] may be cited here. Anomozamites minor (Brongn.) Nath., as restored by Nathorst from specimens from the Rhätic of Scania, with its Williamsonia-like fructifications, Nilssonia-like foliage, and branching habit, is especially to be mentioned in this connection as one of the most interesting fossil plants known. This series is at the same time an exceedingly important one, covering as it does a period extending over much of the Paleozoic to the close of the Jurassic at least, a period so fertile in the evolution of higher forms.
From the Carboniferous to the Cretaceous there is, then, a great and cosmopolitan cy
cadeoid group with simple sparsely inserted leaves. This group is especially in evidence in the Rhätic, a period of marked change in the Mesozoic plant alignment. In the Northern hemisphere the Rhätic is generally notable for megaphyllous forms, especially so in the Richmond and North Carolina coal fields; but, as I found on the eastern side of the Andes in finely stratified Rhätic shales of Argentina, there are many small leafed and even scrub or upland forms. The small extent to which these forms showed netted venation is of course the weakest link in the chain of evidence for dicotyl derivation. But it may be recalled that the presence of net-veined forms [Dictyozamites] is not an inference; while the perfunctory reference of the numerous netveined leaves of the Mesozoic to ferns, rarely found fertile, rests on no sound or certain basis. The great lyrate ferns of the Rhätic, Clathropteris and Dictyophyllum, are an exception; also an adumbration of the higher net-veined types soon to become dominant.
This subject has been considered more at length in a paper in the American Journal of Science (November, 1914). It is there suggested that polar stocks of unknown record may account in part for the seeming gap in the dicotyl leaf series. Also it should be noted that plastic, potent upland stocks (Bailey and Sinnott), are in the strictest sense of the word corollary to the main theory of radiation of animals and plants from the polar areas (Darwin, Forbes, Rütimeyer, Saporta, Joseph Dalton Hooker, Asa Gray in the Dubuque Address of 1875, Narthorst, Wallace, Scribner, Wortman, Wieland and Matthew). In any case the negative evidence indicates small flowers for the early dicotyls from the Permian on, and these are only likely to occur in some favored or unusual locality.
It has been said that the resemblances between Cycadeoid and dicotyl wood structure are deceptive, that some Magnolias (and the Trochodendrons) have lost their vessels, and that scalariform sculpture of the dicotyledonous vessel results from the lateral fusion of circular pits (Jeffrey). Even if the first of these contentions, improbable as this now seems,
were true, it no longer need be held vital. The second has been fully and thoroughly shown by Brown to be at variance with the facts. By the simple process of segmentation of the primitive and more fern-like scalariform pits, the circular-pitted and slit-pitted modern types of vessels must arise. When the scalariform cycads and the Araucaria, and Magnoliaceous (Drimys) seedlings are adequately compared, every stage in this fundamental change, which affords the very basis of dicotyl evolution, becomes visible.
Over and above the collateral evidence now at hand, three notable generalizations of the last few years further affect our conceptions of the fossil record of the dicotyls:
Firstly, there is a very small record of the upland vegetation of past times; although the enormous extent of the unknown upland record could not be surmised so long as the alternate emergence and subsidence of the continental areas remained wholly unmapped. Yet it appears that the high upland and polar, and not the tropic or coastal fringe plants have long included the great majority of plastic forms; and it is certain that upland and polar forms moved forward during the periods of continental emergence closing geologic epochs, and were least liable to extinction during medial subsidence. That is to say, we know best the aplastic coastal fringe forms with a broken record.
Secondly, the fossil record has gradually lengthened out in the case of so many lines both animal and plant, that a nearly universal parallelism comes into view instead of a comparatively recent development from constantly dichotomizing stocks. That is, slowly converging lines replace the "paleontologic tree of the texts; and now that these persistent lines are more clearly discerned they are rapidly being pushed back still further on purely anatomical grounds. Thus it is not only easy to admit the view of Seward that the Araucariales are distinct, but it now seems difficult to show the connection of any of the gymnospermous groups since Carboniferous times. Their resemblances are all in large degree homoplastic.
Thirdly, Bailey has found much evidence for a progressive tracheidal shortening in all woody plants from the Carboniferous on. His fuller results are awaited with the greatest interest. The wonder is that it was not discerned long since that some such general course of change formed the basis of later stem evolution. Now this leads to a very obvious conclusion as to the origin of complexity of structure in dicotyledonous stems. Taking into consideration the observations already made as to the tracheidal origin of vessels, and the general dicotyl stem features, it seems that the main order of dicotyl stem change has been, beginning some time in the Permian: (a) shortening of the tracheids, (b) segmentation of scalariform into circular pitting, (c) pith reduction and development of both medullar and radial storage tissues, (d) secondary [relative] enlargement of short types of circularly pitted tracheids coordinate with [actual] reduction of adjacent elements to the condition of fiber tracheids, (e) progressive radial sclerotization, (f) development of extreme branching in sparse-leafed shoots. In recent interesting and serviceable descriptions of Lower Cretaceous woods by Stopes1 exactly the older features agreeing with the course of change just outlined are sharply in evidence. In the Lower Cretaceous Aptiana the small vessels scarcely disturb the regular radial sequence, are little larger than the wood fibers, and nearly as elongate as normal tracheids. The medullary rays are similarly old of aspect, with considerable variation in cell length and size, the short rectangular form being almost cycadeoid. Fully in accord with the views advanced, scalariform wood is a striking feature in not only Aptiana, but several other Lower Cretaceous genera described by Stopes.
Morphologically speaking a point has therefore been reached where dicotyls are no more isolated than the conifers. From the viewpoint of stem, foliage and flower they are separated from other groups by no impassable gap. In fact the main categories of evidence for dicotyl origin are to be found within the 1 Brit. Mus. Cat. Mesozoic Plants, 1915.