THE VASCULAR CRYPTOGAMS 87 



the rings or spirals of the disorganised tracheae (see 

 Fig. 39). The later-formed part of the xylem, con- 

 sisting of a few scalariform tracheides (not vessels), is 

 usually separated from the protoxylem by a little 

 parenchyma, and forms two groups to the right and 

 left of the bundle. The whole wood therefore, if 

 continuous and not disturbed by the carinal cavity, 

 would form, as seen in transverse section, a V with the 

 point inwards and the limbs outwards. The phloem 

 lies between the limbs of the V (see Fig. 39, ph). It 

 consists of sieve-tubes (with sieve-plates on their oblique 

 transverse walls) and parenchyma. Beyond this, on the 

 outer side, we come to the pericycle and then to the 

 endodermis (with well-marked cuticularised bands on its 

 radial walls) which marks the beginning of the cortex 

 (Fig. 39). The xylem is often very little developed, 

 especially in the rhizomes and the stems of aquatic 

 species. The pith, or what remains of it, when the 

 stem is fistular, consists of ordinary parenchyma, and 

 presents no features of interest. 



The cortex, however, at least in the aerial stems and 

 branches, is highly differentiated, as indeed we might 

 expect, considering that it has here to perform the 

 assimilating function usually assigned to the leaves. 

 The inner cortical layers consist of large - celled 

 parenchyma traversed by the air-containing vallecular 

 spaces. The outer cortex is made up of two kinds of 

 tissue, namely, sclerenchyma, fulfilling the mechanical 

 function of strengthening the stem, and chlorophyll- 

 tissue, to which the functions of assimilation and 

 transpiration belong (see Fig. 39). Now both these 

 tissues need to be as near the surface as possible, 

 in order to do their work to the best advantage. The 



