332 



MANUAL OF BOTANY 



in those leaves from whose upper surface they are entirely 

 absent. 



Stomata in one Square Inch of Surface. 



Though the guard-cells of stomata can usually vary the 

 size of the opening, or altogether close it, according to circum- 

 stances, this is not universally the case. In some plants large 

 stomata occur whose guard-oeUs cannot change their form so as 

 to do this. These, which are known as water -stomata or water- 

 pores, are chiefly concerned in the excretion of water, the ordinary 

 form being adapted to regulate the escape of watery vapour only. 



Hairs or Trichomes. — These are external prolongations of 

 the epidermal cells. Each consists of a foot or part embedded 

 among the other epidermal cells, and a body or part extending 

 outwards. They may be unicellular or multicellular, and either 

 kind may be variously forked or branched. The component 

 cells of multicellular hairs may be variously arranged. Com- 

 monly they are placed end to end in a single row, so that the 

 hairs assume a more or less cylindrical form. Both multi- 

 cellular and unicellular hairs, instead of being erect, may 

 develop horizontally in a more or less circular manner, and 

 form stellate hairs, as in the Ivy, in Deutzia, scabra, &c. ; or 

 the component cells may develop in opposite directions from 

 another cell raised above the level of the epidermis, so as to 

 produce what is termed a shield-like or peltate hair. 



Scales are modifications of stellate hairs. They may be 



