166 THE ELEMENTS OF VEGETABLE HISTOLOGY 



mary tissues is an indication that the material is from 

 an herbaceous or a monocotyledonous stem. Herb- 

 aceous stems may be readily distinguished from those 

 of mature monocotyledons by the character of the 

 fibro-vascular bundles occurring in the specimen. 



CHARACTERS OF THE PRIMARY STEM TISSUES 



Epidermis. — The epidermis of herbaceous stems 

 is a single layer of colorless transparent cells, the 

 walls of which are impregnated with cutin. The 

 individual cells, as seen upon transverse section, are 

 more or less rectangular and fit together very closely. 

 Upon surface view the cells appear rectangular, polyg- 

 onal or irregular in form. The epidermal cells of 

 many plants show wavy or undulating walls which 

 fit into the corresponding walls of surrounding cells. 

 Trichomes and stomata are often present and may be 

 of assistance in identification. In powdered materials 

 the epidermal cells are usually apparent on surface 

 view and are frequently attached to the deeper tissues. 

 In monocotyledonous stems of mature growth the 

 epidermal tissues are so modified as to form a heavy 

 covering layer. This, although differing from the 

 cork layers of secondary stem structure, is very tough 

 and resistant. 



Hypodermis. — The hypodermal tissues of primary 

 stems consist of several layers of thick-walled angled 

 cells. In herbaceous stems, the hypodermal cells 

 which contain chlorophyll are partly concerned in the 

 production of starch. In monocotyledonous stems, 

 the hypodermis persists and forms a covering tissue 

 which reinforces the epidermis. 



