XX. On the comparative morphology of the leaf the vascular cryptogams and gymnosperms

Abstract

The origin of the tendency among the earlier morphologists to draw a sharp distinction between stem and leaf may most probably be traced to the fact that vegetable morphology was first pursued as a science in regions where deciduous trees prevail. Seeing the leaves of so many plants fall off as a whole, while the scar left was almost a direct continuation of the external surface of the stem, doubtless gave rise to the view that the two should be regarded as radically distinct members. As the science of morphology progressed it became necessary, if this distinction were to be maintained, to define more clearly how members belonging to these two categories differ one from another. Various attempts were made by authors to show in what the essential difference consisted, the most notable being that of Hofmeister, who brought forward a number of distinctions, based chiefly upon development. The most essential of these were adopted in that section of the Text Book of Sachs, which deals with the relations of leaves and leaf-forming axes. In the last paragraph (No. 8) of that section, he clearly lays down the principle that “the expressions stem and leaf denote only certain relationships of the parts of a whole— the shoot .” This principle is elaborated in his more recent lectures, in which he writes as follows:— “A typical shoot consists of the leaves and the axis, which however are not really to be regarded as different organs, but fundamentally as parts only of one organ . . . . . In their nature, and as shown by the history of their development, the leaves are fundamentally nothing more than processes, or outgrowths of the axis of the shoot. . . . . ” If we accept these propositions, and I do not see how we can do otherwise, the same method of morphological treatment should be applied to the leaf as is usual in studying the stem. On reading current morphological papers, however, it is very apparent that this is not done. Leaving out of account the use of that adhesive terminology, which constantly revives in the mind the older mode of viewing the leaf, it is still obvious that the treatment of the leaf by modern writers is different from that of the stem. Thus, to take as an example the best of the earlier works on leaf-development, viz., Eichler’s Dissertation on the Development of the Leaf; after defining the Primordial Leaf as the young leaf before internal differentiation, or distinction of external parts, the author goes on to describe (p. 7) how the primordial leaf becomes differentiated into “ two chief parts, which are common to the leaves of all Phanerogams, viz., a stationary zone, which takes no part in the further formation of the leaf, and a vegetative part which forms the lamina with its branches.” The former he names the foliar base (blattgrund), and the products of its development are the sheath and the stipules if present; the latter he designates the upper leaf (oberblatt), which gives rise to the simple or branched lamina. The petiole is also, according to Eichler (p. 8), derived from the upper leaf, though other more recent writers describe it as being intercalated between the two parts. This distinction first drawn by Eichler has recently been revived, and the terminology, with some slight modifications, adopted by Goebel. He has however imposed a very necessary limitation upon its application, viz.: that the two parts of the primordial leaf “are not sharply marked off from one another, but are only to be distinguished by the part which they play in the further growth of the young leaf. ” He has, on the other hand, extended it to the Monocotyledons and Gymnosperms, and occasionally also to certain Cryptogams, in which similar phenomena appear. Thus the distinction of the foliar base and upper leaf has become established in botanical terminology, and it is applied equally to both branched and unbranched leaves.