34 METABOLISM 



SCHIMPER has proved that the amount of water absorbed from the funnel exactly 

 corresponds with that lost in these plants from transpiration, whilst their roots 

 are quite unable to provide water in quantity sufficient for their needs. On the 

 other hand, there are some forms provided with special holdfasts and which 

 have lost their roots altogether. The best known of these rootless Brome- 

 liaceae is Tillandsia usneoides, whose long, grey, tail-like masses occur in tropical 

 and sub-tropical America in such quantities that they actually obscure the 

 foliage of the trees on which they are epiphytic. ' The first beginning 

 of a tuft is, as a rule, the separation of a solitary twig which twists round 

 another branch of the tree ; from it arise numerous lateral twigs, some of which 

 become themselves propagative shoots, although most of them develop quite 

 freely into the air.' The leaves of this Tillandsia form no collecting funnels, they 

 are certainly not arranged in a rosette, but come off individually from the stem 

 and are small and inconspicuous. The whole plant is covered with water- 

 absorbing hairs such as occur on the leaf bases of other forms, and these give 

 it its grey colour. In general appearance, as indeed its specific name, ' usneoides,' 

 indicates, it resembles an indigenous lichen, also a pendent epiphyte from trees. 

 This recalls to us the fact that epiphytes occur in our own climate also, although 

 these are almost entirely confined to plants of low grade, viz. mosses and 

 lichens. The feature in which these plants have an advantage over higher 

 forms and which qualifies them to withstand our dry seasons, is not any 

 specially economical management of the water absorbed, but a capacity for 

 being able to withstand desiccation, a capacity, however, by no means confined 

 to epiphytic forms. These plants may often become so dry that they may be 

 actually crumbled into dust, and that, too, without losing their vitality. As 

 soon as the first drops of rain fall on them and they have absorbed as much 

 as they require, they start life afresh. Perhaps the best examples of this 

 peculiar mode of life are to be found among the crustaceous lichens which 

 grow over the walls of old houses or on bare rock. Such forms often obtain 

 in a few hours or days only, during the course of months, all the water they 

 need for carrying on their vital functions, and in the interval are completely dried 

 up by the sun's heat. Not only is this capacity for resisting desiccation of 

 the very greatest importance to these plants, but the ability they also possess 

 of absorbing the first traces of water after long drought is of deep significance. 

 Their cell-walls in the dry condition remain capable of being easily wetted, 

 and rapidly take up water once more, differing in this respect from the dust 

 of our streets, which in losing water loses also its capacity for quickly reab- 

 sorbing it. Owing to this characteristic, mosses and lichens play a very 

 important part in the economy of nature, inasmuch as they are able to store 

 up rain, forming living water reservoirs whose contents are for a long time 

 of benefit to other organisms. Without going into further details we may, in 

 conclusion, merely allude to the fact that other epiphytes which are unable 

 to withstand desiccation, are at first entirely absent from regions subject to 

 periodic deficiency of water. Further they are compelled to exercise great 

 economy with the store of water which they have collected during the rainy 

 period, i. e. they must limit their transpiration greatly or provide themselves 

 with special water reservoirs. Many varieties of such reservoirs have been 

 described by SCHIMPER and GOEBEL, as well as by other earlier writers. 



Bibliography to Lecture III. 



FREIDENFELT. 1902. Flora, 91, 115. 



GOEBEL. 1886. Ber. d. bot. Gesell. 4, 249. 1887. Bot. Ztg. 45, 717. 



GOEBEL. 1889. Biologische Schilderungen. Marburg. 



HABERLANDT, F. 1877. Wiss.-prakt. Unters. auf d. Gebiete. d. Pflanzenbaues, 



2, 158. 

 HALES. 1748. Statik der Gewachse. Halle. 



