ELM STEM 



55 



parenchymatous cells with intercellular spaces : a peripheral 

 part of it may be distinguished as a band of not very definite 

 collenchyma from the thinner walled cortex within. According 

 to their various characters they may be thus grouped : 



a. Ordinary parenchyma cells, with cellulose walls and 

 protoplasmic contents with nucleus, chlorophyll, and starch- 

 granules : the two latter are not constantly present. 



b. Cells (idioblasts) with large crystals. 



c. Large cells whose mucilaginous walls almost or entirely 

 obliterate the cell-cavity. 



Note that the cells (a) are subject to radial division, and 



Fiu. 5- A, B, Diagrams illustrating the formation of periderm in the layer of cells 

 (2) directly below the epidermis (i). A shows the first periclinal division of 

 the hypodermal layer (2). B shows as the result of repeated periclinal divisions 

 a radial row of cells, of which the outer portion (a) is of cork ; the inmost portion 

 (c) is the phelloderm : these are separated by a single cell (/)) which represents 

 the cork -cambium. 



that the whole cortical tissue undergoes tangential extension, 

 so as to keep pace with the increasing bulk of the internal 

 tissues. 



In good sections of young material an endodermal layer 

 may be distinguished running peripherally in a wavy line 

 round the outer margin of the series of bundles. 



4. The sclerenchyma consists of cells with walls so thick- 

 ened that the cell-cavity is often obliterated : the walls are 

 differentiated into two or more strata. Reactions with 

 aniline sulphate, light yellow ; with chlor-zinc-iodine, brownish 

 red. 



5. The soft bast is, as in the Sunflower, composed of 



