28 MORPHOLOGY OF THE ANGIOSPERMS 



Reduction in the cotyledon of the two traces to one — as in stems — has 

 been described in Scilla (Chap. 9). 



Recognition of the two-trace node as primitive for angiosperms con- 

 stitutes a major forward step in the use of anatomy in interpreting the 

 phylogeny of the angiosperms (although odd-numbered traces will 

 doubtless continue to be described as characterizing angiosperms). In 

 the possession of a nodal trace system based on two units, the angio- 

 sperms join the other megaphyllous taxa. 



In angiosperms, an cndodermis is present in roots and in the stems 

 of many herbaceous taxa and of seedlings. In these locations it is often 

 apparently vestigial. It consists of a layer of cells resembling endo- 

 dermal cells in form and arrangement but lacking the critical character 

 of cutinized wall areas. The stems of woody plants lack an cndodermis. 



The presence in the angiosperms of a perici/cle, a sheath of tissue be- 

 tween the vascular tissue and the cndodermis, has been questioned. It 

 has been shown by critical histological studies that, in at least some 

 genera, the fibers between the phloem and the cndodermis, commonly 

 called "pericyclic fibers," belong to the primary phloem; protophloem 

 elements formed among them soon degenerate and disappear as the 

 stele matures. The morphological problem here relates to paren- 

 chyma cells that often lie between these fibers and the cndodermis. 

 These may be part of the protophloem or of the more or less distinct 

 pericycle. The absence of an cndodermis in most angiosperm stems makes 

 the delimiting of the primary phloem, with its parenchyma and fibers, 

 uncertain. A pericycle is characteristic of vascular cryptogams and of 

 the roots of seed plants. In the angiosperm stem, the pericycle, together 

 with the cndodermis, is in process of reduction and loss. 



The primitive angiosperm leaf, under the interpretation of the primi- 

 tive nodal structure as trilacunar, was seen as palmately veined, per- 

 haps three-lobed, with three major veins united at the base of the 

 blade. The present recognition of the unilacunar, two-trace node as 

 apparently the basic nodal structure also supports the concept of the 

 palmately veined leaf as primitive. Leaves with two traces continued 

 independently through petiole and blade (as a double midvein) are 

 rare — Austrobailcya (Fig. 5), Chloranfhus, Sarcandra, Ascarina. Pairs 

 of traces that continue through the organ are more frequent in cotyle- 

 dons and sporophylls than in leaves (Fig. 7). 



The more advanced three-trace system is formed by the addition of 

 two lateral traces with separate gaps (Fig. 6C). The three traces — the 

 median double in nature fundamentally — unite at various points: in the 

 cortex, at the base of the petiole, or in the petiole. From this three- 

 trace system, a common type, have been derived other still more 

 specialized types. 



