Glandular Trichomes. 49 



could reach the flowers of the shoots standing just 

 over the edge, whilst they carefully avoided pressing 

 forward over the fluid mud in the tub in order to reach 

 those shoots which stood up in the middle. 



3. Access to Flowers impeded hy Viscid Secretions. 



I have already mentioned that trichomes, of a 

 special kind, which secrete a viscid substance, are 

 developed on the epidermis of Polygonum amfhi- 

 hium L., when this plant is not surrounded by water 

 but grows on land from which the water has been 

 dried up ; and that these glandular trichomes act as 

 a protection to the flower from the visits of creeping 

 insects. This form of protection is one that occurs in 

 plants with special frequency, and is an excellent 

 defence to their nectar, not only from creeping insects, 

 but also from unwelcome flying ones.^ 



It is always epidermal structures that furnish the 

 viscid substance by which the path to the flower, or 

 to certain parts of it, is made impassable for disadvan- 



* The viscid substances with which parts of plants are coated 

 are not always intended to act as a protection to the flowers from 

 unprofitable visits. The viscid coating of many young leaves, 

 especially in plants whose flowers are nectarless, and whose pollen 

 is dusty (Populus, AJnus, Betula, Juglans), protects these leaves 

 while stUl young from evaporation and desiccation, acting as a 

 layer of varnish. In many cases also the sticky secretion protects 

 the foliage from herbivora. That the viscid coverings also fre- 

 quently facilitate the dispersal of the seed by means of animals 

 has been already mentioned on an earlier page (p. 10), 



