948 A TEXTBOOK OF THEORETICAL BOTANY 



namely that its vascular supply leaves a gap in the vascular system of the stem, 

 which stamps it as belonging to the same megaphyllous cycle of affinity. 

 This structural condition is called phyllosiphonic, in contrast to the 

 cladosiphonic condition in which only the branch-traces leave gaps in the 

 stem stele, a condition which is characteristic of the Lycopodiales and other 

 microphyllous groups. In Chapter XVI we have seen that the megaphyllous 

 leaf may have been evolved from a flattened branch-system bearing micro- 

 phyllous leaves, whose small separate laminae became merged into a larger 

 general lamina. This evolutionary scheme, which is associated with the 

 names of Lignier, Tansley and others, harmonizes with the apparent 

 derivation of both microphyllous and megaphyllous stocks from primitive, 

 leafless types like the Psilophytales, with dichotomous axes, (see p. 650) and 

 it has much to commend it. Among the Psilophytales we find types which 

 are covered with minute leaves, developed apparently like superficial 

 emergences, which invite comparison with the microphylls of both Mosses 

 and Lycopods. If we accept the homology of these leaf types we must 

 regard the microphyllous state as the more primitive of the two and the 

 megaphyllous state as derived from it. 



From the pteridophytic leaf, through the gymnospermic to the angio- 

 spermic leaf, the fundamental type has not altered, though specialized growth 

 patterns have brought about many changes of form. One rather marked 

 difl'erence may be observed in the vein systems, or venation. In Ferns an 

 " open " dichotomous venation is characteristic, in which the branching of 

 the veins is open towards the leaf margins, while in Angiosperms the lamina 

 is usually pervaded by a vein-network with closed meshes. We shall return 

 to this subject later (p. 957). 



We have previously quoted examples of both rootless and stemless plants, 

 but if we consider the formative importance of the leaf in the architecture of 

 the stem, which we have outlined in the last chapter, it is scarcely surprising 

 that the existence of a truly leafless plant is doubtful. There are of course, 

 very many plants in which the leaves have been reduced until they are no 

 more than scales, yet these cannot be considered as other than metamorphosed 

 leaves. Scales may be reduced almost to invisibility, as on the rhizome of 

 the saprophytic Orchid CoraUorhizo innata, or they may have lost even a 

 vascular supply, as in Cuscnta, and consist only of undifi^erentiated 

 parenchyma, but they are, nevertheless, so clearly linked by intermediate forms 

 with larger and better developed leaves, that we must regard them as leaves 

 in spite of their extreme reduction. If we exclude floral leaves and speak 

 only of vegetative structures, the nearest approaches to a truly leafless 

 condition are to be found in the following : (i) the early stages of the parasite 

 Orobanche, which has no vestige of leaves or of an apical bud, until it has 

 become well established on its host plant (Fig. 936) ; (2) the cylindrical green 

 stems of HeJeocharis and Scirpiis lacustris, which consist of a single, greatly 

 extended internode, bearing only (in the sterile state) two small scales at the 

 apex ; (3) the cladodes of Asparagus, if so extreme a case may be admitted, 

 can also be cited as leafless stems, though of a very reduced kind. 



