GENERAL NUTRITION 231 



c. FOLIOSE LICHENS. Though many of the leafy lichens are provided 

 with a tomentum of single hyphae, or with rhizinae on the under surface, 

 the principal function of these structures is that of attaching the thallus. 

 Sievers 1 tested the areas of absorption by placing pieces of the thallus of 

 Parmeliae, of Evernia furfuracea, and of Cetraria glauca in a staining 

 solution. After washing and cutting sections, it was seen that the coloured 

 fluid had penetrated by' the upper surface and by the edge of the thallus, 

 as in crustaceous forms, but not through the lower cortex. 



By the same methods of testing, he proved that water penetrates not 

 only by capillarity between the closely packed hyphae, but also within the 

 cells. A considerable number of lichens were used for experiment, and 

 great variations were found to exist in the way in which water was taken 

 up. It has been proved that in some species of Gyrophora water is absorbed 

 from below: in those in which rhizinae are abundant, water is held by them 

 and so gradually drawn up into the thallus; the upper cortex in this genus 

 is very thick and checks transpiration. Certain other northern lichens such 

 as Cetraria islandica, Cladonia rangiferina, etc., imbibe water very slowly, 

 and they, as well as Gyrophora, are able to endure prolonged wet periods. 



That foliose lichens do not normally contain much water was proved by 

 Jumelle 2 who compared the weight of seven different species when freshly 

 gathered, and after being dried; he found that the proportion of fresh weight 

 to dry weight showed least variation in Parmelia acetabulum, as ri4 to i ; 

 in Xanthoria parietina it was as T2I to I. 



d. FRUTICOSE LICHENS. There is no water-conducting tissue in the 

 elongate thallus of the shrubby or filamentous lichens, as can easily be tested 

 by placing the base in water: it will then be seen that the submerged parts 

 alone are affected. Many lichens are hygroscopic and become water-logged 

 when placed simply in damp surroundings. The thallus of Usnea, for 

 instance, can absorb many times its weight of water: a mass of Usnea 

 filaments that weighed 3-8 grms. when dry increased to 13-3 grms. after 

 having been soaked in water for twelve hours. Schrenk 3 , who made the 

 experiment, records in a second instance an increase in weight from 

 3'97grms. to in 8 grms. The Cladoniae retain large quantities of water in 

 their upright hollow podetia. The Australian species, Cladonia retepora, the 

 podetium of which is a regular network of holes, competes with the Sphagnum 

 moss in its capacity to take up water. 



To conclude : as a rule, heteromerous, non-gelatinous lichens do not 

 contain large quantities of water, the weight of fresh plants being generally 

 about three times only that of the dry weight. Their ordinary water content 

 is indeed smaller than that of most other plants, though it varies at once 

 with a change in external conditions. It is noteworthy that a number ef 

 1 Sievers 1908. 2 Jumelle 1892. 3 Schrenk 1898. 



