BOTANICAL GAZETTE. , 182 



ner surface had expanded, under these changed conditions, into cor- 

 pulent psei'do trichomes with irregularly thickened walls. 



Still more remotely related to true trichomes are the hernioid 

 cells* found in the large vessels 1 f the older roots. They are formed 

 by the local surface growth of a eel! wall lying in contact with a pitied 

 duct, so that the cell-wall, aided by pressure of the cell content.^;, is 

 forced through the opening of the pit into the cavity of the duct, where 

 it makes a bladder like expansion. 



Ovules are in some species of plants considered to be metamor- 

 phosed trichomes. But such is evidently not the case in this instance, 

 for the reticulated venation, characteristic of Cucurbitaceous seeds, 

 shows them to be unecjuivocably homologous with a portion of the 

 leaf-blade. 



All the forms mentioned in this article fall under one of twoclass- 

 es, capitate or filiform. Trichomes of the former class are inclined to 

 be glandular while those of the latter are not. This accords well with 

 their distribution over the plant surface. The iiliform ones contain 

 the more highly vitalized protoplasm, as manifested by its activity. 

 Tliey are situated on the parts of the plant which are destitute of stom 

 ata and growing ra])idly, and consequently in need of some other 

 means of directly absorbing water and oxygen from the atmosphere 

 and soil. On the other hand the capitate forms are on surfaces well 

 provided with breathing-pores. The latter serve to absorb oxygen 

 during the earlier stages of growth, while the stomata are inefficient. 

 When the stomata perform their all 'tted function, and the intercellular 

 spaces are free ot sap and protoplasm, these capitate trichomes become 

 glandular and are turned to other service, or disappear. 



Considered physiologically, trichomes are not indispensable to the 

 plant, yet in a small way frequently render important assistance. The 

 value of the root hairs is one of the best known ftcts in Botany, being 

 uniformly illustrated in general treatises, and insisted upon in horticul 

 tural essays on transplanting. Hairs on many plants serve for protec- 

 tion against detrimental changes of the weather, the attacks of animals, 

 etc , but in Echiiiocystis the only trichomes that could be considered 

 protective are those which roughen the leaves. Upon rapidly grow- 

 ing parts (especially true, e. g., of young flower buds) the abundance 

 of delicate trichomes aids in supplying oxygen to the tissues. Stomata 

 and air cavities perform this office in the older portions, but in parts 

 newly formed the cavities are filled with protoplasm and cell sap, so as 

 to prevent free circulation of air. Moreover, a very rapid supply of 

 OKygen is required at this time to meet the needs of metastasis by 

 which the increase of jjrotoplasm and the formation and multiplication 

 of cells is effected. The oxygenation of the plant in such growing 

 parts is somewhat analogous to that of some polyps and worms having 

 external filamentous gills, while later it assumes the more efficient in- 

 ternal respiration corresponding to that of insects. 



It has already been hinted that there is a similarity of function be- 

 tween the hairs of growing shoots and of young roots. Both subserve 



♦Illustrations of these arc given in Bcssey's Botany, p. HO. 



