CHAPTER IV. PLANT-HAIRS OR TRICHOMES. 43 



producing pressure upon the poison sacs at the base and in- 

 jecting the irritant liquid into the wound (see Figs. 141 and 1^2). 

 The glandular hairs of the Tobacco, and of the Sage, Fig. 145, 

 the bristly hairs of the Canada Fleabane, the stellate hairs 

 of Solanum elseagnaefolium, Figs. 156 and 157, and of related 

 western species, the branching, glutinous hairs of the common 

 Mullein, Fig. 154, and doubtless the dense, hairy coverings of 

 many other plants, efficiently protect them from the ravages of 

 many destructive insects. It is reasonable to suppose, also, that 

 the dense covering of hairs often found on the under surface of 

 leaves, may, besides guarding these very vulnerable parts of the 

 plant's surface from insect enemies, serve to prevent dust parti- 

 cles from lodging upon and clogging the stomata, and, what is 

 of much greater importance, protect them from the spores of 

 destructive fungi, which otherwise would often find a lodgment 

 in the stomata, and germinating, penetrate the leaf, destroying 

 its tissues and endangering the life of the plant. This view is 

 the more probable since we know that many of the more destruc- 

 tive moulds have no power to penetrate the leaf except through 

 the stomata. 



In connection with the protective influence of hairs, it is 

 significant that the young and tender leaves of many plants are 

 well covered with glandular, or other hairs, while the mature 

 leaves of the same plant are entirely free from them. Some 

 species of Oak, Hickory and Rhododendron afford examples. 

 In the latter plant, the young leaves are very glutinous from the 

 presence of club-shaped, many-celled glandular hairs, one of 

 which is illustrated in Fig. 158. The mature leaves do not pos- 

 sess them. 



In some species of Thistles and other Compositae, the gland- 

 ular hairs borne on the involucre, and on the stalk of the in- 

 florescence below it, are serviceable in preventing ants and 

 other wingless insects from gaining access to the flowers and 

 robbing them of their sweets and so preventing the visits of 

 winged insects that would be useful in transferring the pollen 

 from one flower to another, and so effecting cross-fertilization, a 

 matter of no little importance to plants, as we shall hereafter 

 see. The glandular secretions on the upper internodes of the 

 Catch-fly perform a similar service. 



