196 



The Phenomena of Morphogenesis 



Fig. 8-7. Partition walls opposite adjacent ones. At left, dividing cells in tissue of 

 wounded petiole of Bryophyllum, the walls being laid down directly opposite those 

 in adjacent cells. At right, similar divisions in more mature tissue below wound 

 surface. ( From Sinnott and Block. ) 



in the transverse section of certain roots, where the inner cortex is radially 

 concentric, with opposite walls, but the outer cortex shows the alter- 

 nating arrangement typical of ordinary parenchyma ( Fig. 8-8 ) . 



Endodermis. One of the simplest of these tissue patterns is shown by 

 the endodermis. This is a single layer of cells differentiated in a specific 

 way, as by special thickenings in the walls or the presence of a Casparian 

 strip. It separates the vascular cylinder from the cortex. The position 

 it occupies is usually a very definite one, and in such plants as Equisetum 

 its particular pattern with reference to the bundles is specific enough to 

 be valuable for taxonomic purposes. 



Fig. 8-8. Transverse sections of two roots. Left, Sporobolus, in which there are three 

 layers of radially concentric cells in the cortex, the walls having been laid down 

 directly opposite those in adjacent cells. Air spaces later appear at intersections. Right, 

 Agrostis, in which the cell walls in the cortex always avoid adjacent ones. (From 

 Sinnott and Bloch. ) 



