[64 



THE STRUCTURE OF ECONOMIC PLANTS 



nodes where the vascular bundles form a lignified diaphragm. 

 There are five principal tissue systems: (i) epidermis, (i) mechani- 

 cal tissue, (3) chlorophyll parenchyma, (4) fundamental paren- 

 chyma, and (5) vascular bundles. (Fig. 73; see also Fig, 24.) 

 Aside from the guard cells of the stomata, there are two types 

 of epidermal cells, one of which is narrow and elongated, the 

 other short and isodiametric. The elongated cells are arranged 



in longitudinal rows, sep- 

 arated at intervals by the 

 smaller cubical ones; and 

 the width of the cells va- 

 ries, those which adjoin 

 the mechanical tissue of 

 the hypodermis being nar- 

 rower than those outside 

 the chlorenchyma. The 

 walls are much thickened 

 and there is a well- 

 developed cuticle on the 

 portions which are not 

 covered by the leaf sheath. 

 The stomata occur in 

 single or double rows and 

 are similar to those found 

 in the leaf. In varieties 

 which the stem 



in 



IS 



Fig. 73. A transection of a sector of the stem : cav 

 central cavity of the stem ; chl par, chlorophyll paren- j u U 



chyma; ^ c, guard cells ; w^ec/i, mechanical tissue ; mx, grOOVecl, tnere may DC 



metaxylem ; par, parenchyma ; ph, phloem ; px, proto- short SCabtOUS hairs dcvel- 



^^^"' Oped along the furrows. 



The mechanical tissue of the hypodermal region is made up 

 of elongated, fibrous cells with thick walls and narrow lumina, 

 which form a continuous zone of variable thickness within the 

 epidermis; and this encloses the longitudinal bands of chloren- 

 chyma, as well as the smaller peripheral bundles of the vascular 

 system. (Fig. 74, A.') The chlorenchyma abuts the epidermis 

 directly, consisting of thin-walled cells that are circular in tran- 

 section, elongated in the axial dimension with somewhat sinuous 

 longitudinal walls. The rows of stomata are so located that the 

 substomatal cavities open into the parallel bands of chlorenchyma. 

 These are frequently arranged in pairs and adjacent bands may 



