616 ECOLOGY 



" long moss," a rootless plant with minute leaves (fig. 903). In these epiphytes the 

 chief organs of absorption are specialized hairs, with which the long moss, for ex- 

 ample, is completely covered; that these plants do not depend upon root absorp- 

 tion is shown by their occasional development on telegraph wires, and by the fact 

 that they blow from place to place, continuing their activity wherever they lodge. 

 Probably the long moss represents an extreme epiphytic form that has been de- 

 rived from an ancestry similar to its relative, the pineapple; even now there ex- 

 ists a series of intergrading forms, those at the pineapple end of the series having 

 ordinary absorptive soil roots and scattered or localized absorptive scales, while 

 those at the Tillandsia end have only anchorage roots or no roots at all and have 

 an abundant development of absorptive scales (fig. 969). 



The absorptive scales or scale hairs of the Bromeliaceae are epidermal struc- 

 tures consisting of a sunken, multicellular, thin-walled, living stalk, capped by 

 a protruding, multicellular, shieldlike organ, whose cells are dead and have thick 

 cellulose walls (figs. 904, 905). Liquid water is absorbed by the dry scale hairs 

 with remarkable rapidity, but the rest of the epidermis is highly cutinized and im- 

 permeable. When the surface is moistened, the dead cells fill with water, causing 

 an expansion of the structure which lifts it from the rest of the epidermal surface, 

 thus making possible the movement of water along the capillary passages between 

 the scale and the subjacent epidermis, as well as through the scale. The water then 

 enters the living stalk cells osmotically, as in root hairs, the presence of sugar in the 

 cells facilitating the process. When evaporation begins, the dead cells lose their 

 water and the scale collapses, thus closing the region where capillary water enters, 

 and reducing the amount of water lost. It has been shown that salts as well as 

 water may enter these plants through the scale hairs. The Bromeliaceae furnish 

 the only conspicuous well-attested example among the higher plants of the absorp- 

 tion of water by aerial leaves, and the only well-attested structure in any group, which 

 both facilitates absorption and retards transpiration. Nothing is known concern- 

 ing the steps in the evolution of these hairs, and their rigidity gives little hope of 

 obtaining through experiment a clue to their origin. 



Food absorption by the leaves of carnivorous plants. The sundews. 

 A few plants possess the power of absorbing and digesting animal food. 

 The best understood of these is the sundew, Drosera, a bog plant whose 

 leaves have prominent glandular hairs, which usually are wine-red in 

 color and tipped with viscid secreted drops that glisten in the sunlight 

 like dewdrops (fig. 906). Insects, which make chance visits or which 

 are attracted by the leaf brilliancy or color, are held by the viscid 

 drops, and their efforts to escape result in contact with other drops, so 

 that they are held still more securely. The presence of the insect stim- 

 ulates the glandular hairs to secrete more actively and also differently, 

 the more important substances secreted being formic and other acids 

 and proteolytic (i.e. protein-digesting) enzyms, which transform into 

 solutes the digestible portions of the insect body. The presence of the 



