32 



THE PLANT CELL. 



root-hairs (tricliomes), where the ectoplasm forms a distinct layer 

 which exercises a marked selective capacity over the absorption 

 of salts in the soil (see Fig. 21) ; but the function of most simple 

 hairs is in the main one of protection either from excessive cold, 

 heat, or mechanical injury. Occasionally a large number of hairs 

 are aggregated together to form one variety of emergence ;* and 

 in the case of the long felt-like hairs which cover buds (Hazel 

 and Alder), these are mainly useful in protecting the latter from 

 the effects of frost. Simple hairs may at times be branched 

 (stellate hairs). 



i~l( — ir 



Fig. 17. — A Compound Hair from Rhododendron— 3,\j the base are six 

 •-small cells. The small projections on the wall of the large upper cell 

 are composed of carbonate of lime. 



Multicellular hairs are those which, having retained their 

 protoplasm, divide and form several cells lying in one or more 

 planes (see Figs. 1 7 and 1 9) ; such hairs may be stellate, sickle- 

 shaped, or shield-shaped. Stellate hairs may, however, be a 

 variety of branched unicellular hair ; and in the case of the 

 long simple hairs, a wall may arise which cuts off the elongated 

 portion from the original epidermal cell (see Fig. 16). 



* These emergences may also have cells from the deeper layers in their 

 structure ; some of them are glandular and possess an internal secretory 

 layer. 



