HYGROSCOPISM. 289 



structural adaptation to secure a certain end, which 

 is beneficial to the plant. Surely it must be too 

 delicate a distinction to admit specialised structure 

 in other instances, such as stigmatic surfaces, &c, 

 and reject it in this. As the opening and closing of 

 the peristome takes place whilst the plant is living, 

 even whilst the urn of the plant is still attached and 

 living ; and as it is of a manifest utility in securing 

 the dispersion of the spores at such a period as when 

 the moisture of the atmosphere would best secure 

 their germination, we are prepared to retain the 

 peristome of mosses as a satisfactory illustration of 

 hygroscopism. 



In the Liverworts (Hepaticce) the spores are mixed 

 in the capsules with spiral threads, or elaters. If the 

 contents of one of these capsules are moistened after 

 they have become dry, the spiral threads will be seen 

 wriggling and twisting about, by means of the re- 

 laxation of the spiral, such movement also being 

 of assistance in the dispersal of the spores. In the 

 same manner we have observed the threads in such 

 Myxomycetes as Trichia, in which the threads are 

 spiral, relax a little when moistened after they have 

 become dry, but, in this instance, only to a limitec 

 extent. 



The examples we have given are sufficient to show 

 that, inasmuch as there are movements in plants 

 which result from the influences of light, tempera- 



U 



