i8 



THE STRUCTURE OF ECONOMIC PLANTS 



tubular structure with thin cellulose walls. The cytoplasm forms 

 a thin layer immediately within the wall, and the nucleus may 

 frequently be located in the extended portion of the cell. With 

 few exceptions, the hairs are short-lived, soon collapse, and the 

 basal portion may become suberized or lignified. New hairs are 

 formed continuously so that a definite zone of root hairs is main- 

 tained directly back of the elongating portion of the root axis. 



Fig. 6. A, a portion of the epidermis of the celery petiole showing a stage in the develop- 

 ment of the stoma; B, the epidermis of the blade showing stomata, guard and accessory 

 cells: ac, accessory cells; wc, mother cell. 



Shoot Hairs. — The numerous epidermal hairs produced on the 

 stem, leaf, and inflorescence are variable as to form and structure. 

 In many instances, they appear to have little functional significance, 

 but the abundant production of hairs on the exposed surfaces of 

 aerial organs does result in a pubescence which may be protective. 

 Characteristic glandular hairs are developed in Cannabis and 

 Lycopersicum ; and, in Lactuca, stigmatic hairs occur which are 

 related to the process of pollination. Many plants, including 

 Cucurbita and Solanum, have multicellular hairs which may or may 

 not be capitate, and unicellular hairs are common in Medicago and 

 Cannabis. A type of special economic interest is the fibrous hair 

 which develops from cells of the epidermis of the ovule in Gossyp- 

 ium. (Chapter XIV, Fig. liy.) 



