130 THE ANATOMY OF WOODY PLANTS 



number of instances the accessory cells lie over the guard cells, 

 so that the latter are nearly or quite shut off from the surface of the 

 leaf. 



In many cases the guard cells have walls which differ chemically 

 in different regions. In the pine, for example, the walls of the 

 elements inclosing the pore of the stoma along their inner margins, 

 where they actually abut on the stomatic aperture, and on the 

 opposite sides, where they are in direct contact with the adjacent 

 cells of the epidermis, are in a condition of pectic cellulose and 

 absorb strongly certain of the hematoxylin stains. It has been 

 quite generally observed in physiological investigations that the 

 inner and outer walls of the stomatic guard cells are extremely 

 viable to water so much so, hi fact, that a deficient water supply 

 in plants flourishing under ordinary garden conditions results in 

 the rapid loss of turgescence in these elements. 



The epidermal structures are frequently characterized by out- 

 growths known as hairs. These are most generally composed of 

 comparatively few cells, but in some instances may become very 

 complicated in their organization. When it is clear, in spite of any 

 degree of elaboration and magnitude, that the organs are purely 

 epidermal hi their origin, they are properly designated as hairs 

 or trichomes. If tissues of the fundamental or fibrovascular 

 systems, or both together, enter into the structure of the processes, 

 they are known as emergences, provided they do not come more 

 accurately under the caption of modified leaves or lobes of leaves, 

 or, finally, of branches. The trichomes are not usually of a high 

 value hi connection with evolutionary anatomy, since they are 

 subject to a very considerable degree of variability in accordance 

 with conditions of environment. 



The sporangia of vascular plants are sometimes considered 

 to belong in the category of trichomes, but it is very difficult to 

 bring them consistently under this heading. While it may be 

 maintained with a certain degree of force that the spore sacs of the 

 polypodiaceous ferns are of the nature of hairs, since they are 

 clearly entirely derived from the superficial cells of the leaf, it is 

 difficult to homologize them on this interpretation with the spore- 

 producing members of the lower and more primitive groups of 



