1899.] ESSAYS. 47 



very pretty. The voiiiiug Jibove tilluded to is contined to a 

 midrib, sometimes only a beginning at the base of the leaf, 

 sometimes extending half way, or just short of the a[)ex or 

 beyond. It has been shown to be mainly a conduetor of water, 

 and is found chieHy in leaves of plants whose habitat is such 

 that it has a constant supply of moisture. In these conditions, 

 it has been found that a well-developed costa will supply the 

 loss of transj)] ration. The property which mosses have of dry- 

 ing up and again reviving is a very remarkable one. It resides 

 in an undetected {)roperty of the protoplasm. How remarkal)le 

 that })roperty is is proved by some artiticial experiments. 

 Severe desiccation in a drier was applied to Barlmla-muraJis 

 with the aid of sulphuric acid, and after eighteen months, after 

 a few wettings, it renewed growth in all its parts. A species 

 of Griinmia was dried until it would pulverize, left in a drier 

 ninety-five weeks, and again revived. This is in excess of any 

 natural drying it could be subjected to. Mosses have developed 

 many devices to economize moisture and to distribute it rapidly 

 over the whole plant, so that a short, small supply of rain will 

 do the most good, by being spread and retained. 



Some, like Dirianum-undulaiutn and the Mniums have a 

 felting of hairs about the lower part of the stem, ascending 

 more or less. The leaves are arranged overlapping or clasping 

 about the stem so as to form a system of capillary chambers 

 through which the water passes rapidly. In Sphagnum there 

 are two sorts of cells, — one of large green cells, in which the 

 assimilation is done, and long capillary cells which are colorless 

 and furnished with points or pores for the rapid circulation and 

 storing of water. 



It is owing to the capacity of holding moisture and rapidly 

 gathering it that makes mosses so valuable in the forest floor or 

 on open plains, especially on mountain slopes, as they prevent 

 the rapid flowing of the water, which carries away the soil. I 

 was reading recently an account of large areas in Alaska covered 

 deeply with mosses, which prevented the gullying and denuding 

 of these districts. 



An interesting point is that in some of the higher mosses, or 



