WETMORE: LEAF-STEM RELATIONSHIPS 25 



division of labor has occurred, the leaf being set off physiologically from the 

 axis bearing it even though it originated as a product of the same meristematic 

 activity which adds to the stem tip? There is increasing evidence that in 

 many cases in diverse groups of Vascular Plants each foliar primordium so 

 produced is provided with procambium continuous from below at all times 

 (Esau, 1943; Wetmore and Smith, 1942, etc.). Whatever the later orderly 

 differentiation of primary xylem and primary phloem may be, that continuity 

 seems of significant import. The boundary then between leaf and axis is 

 indefinite with leaf buttresses present as those parts of the axis from which 

 leaf primordia are elevated. It must be pointed out, however, that the 

 "influence" of the leaf is of different degrees in different groups of plants. 

 Certainly in roots, in rhizomes of Lycopodium ohscurum, in leafless axes of 

 Nephrolepis, and in the leafless shoot of Pseudotsuga, cortex is produced 

 as well as a vascular cylinder. In the cauline structures, the cortex is ordinarily 

 retarded in its development. In Lycopodium and Selaginella, microphyllous 

 plants, cortex is ordinarily slow in developing. In the Conifers, one finds 

 needle-like or scale-like small leaves and a slowly developing cortex. In the 

 Angiosperms and Ferns with their characteristic large leaves, cortical and 

 vascular differentiation is early, yet small-leaved types such as Liniim show 

 the usual delay (Esau. 1942). As Kaplan (1937) has suggested, cortex 

 appears soonor or later but leaves seem to accelerate the process of cortical 

 vacuolation. 



If I, this early, should venture to epitomize the leaf-stem situation, it would 

 be something as follows : The early, psilopsid land plants, still leafless, were 

 protostelic. With the advent of leaves, microphyllous or megaphyllous, various 

 changes have occurred in stem organization. Microphyllous plants possess 

 in their primary a:xes a small amount of "foliar" trace vascular tissue, periph- 

 erally connected to the cauline, vascular cylinder. In megaphyllous groups, 

 the foliar vascular system and its stem connections become more significant 

 and the potential cauline portions, failing in varying degrees to differentiate, 

 appear as pith. The shoot system is the sum total of foliar and cauline expres- 

 sion. From the practical point of view, the shoot is still composed of leaves 

 borne on a stem system. From a developmental point of view, where one of 

 necessity is faced with factors underlying development, an understanding of 

 the varied developmental patterns of shoot expression in the vascular plants 

 seems significant. Flow else can one approach experimentation to determine 

 the underlying physiological and biochemical background than with a knowl- 

 edge of the structural and developmental variables? 



The Biological Laboratories 

 Harvard University. 

 Cambridge, Massachusetts 



