THE STRUCTUBE OP THE STEM 



367 



In aquatic stems, both of this and of the monoootyledonous 

 type and of certain Cryptogams, there is comparatively little de- 

 velopment of vifood. Instead, the cortex is unusually large, and 

 frequently shows large lacunae or air-passages, which occupy a 

 large part of its substance, very few rows or chains of cells sepa- 

 rating them (fig. 749). In some cases the cells abutting on these 

 air-passages are developed into hairs of curious form. These 

 laounse are interrupted at the nodes, where the parenchyma is 

 continuous. 



MoNOOOTYLEDONorrs Type. 



The structure of the growing point of stems of this type 

 differs slightly from that of the former one. The apex is 



Fig. 760, 



^. 750. Growing point of stem of monoootyledonous plant. The thickness 

 of the cell- walls at the limits of the dermatogen andperiblem is exaggerated 



for the purpose of distinction. After Douliot. Fig. 751. Diagram of 



monoootyledonous stem, showing bundles scattered more or less irregularly 

 in the stele. 



covered by a dermatogen, within which are the periblem and 

 plerome, but the distinction between the latter two is not weU 

 marked, a single initial layer being probably common to both 

 (fig. 750). 



A transverse section taken a little lower down shows the 

 central stele differentiated as in the former case. The arrange- 

 ment in the stele is, however, very different. Instead of a ring 

 of procambium bundles arranged round a central pith, many 

 bundles are found, arranged more or less in a number of rings, 

 and all pointing to the centre of the stele. Those first formed 



