CELLS AND CELL PRODUCTS 215 



pustulate to be 4^46 per cent., in Usnea barbata 179 per cent., in U. longissima 

 considerable quantities while in Roccella tinctoria it occurred in great abun- 

 dance. It was also abundant in Diploschistes scruposus, 28*17 per cent; it 

 declined in Variolaria (Pertusaria) dealbata to 777 per cent., in Cladonia 

 rangiferina to I76-2'I2 per cent, and in Ramalina fraxinea to r8 per cent. 



Calcium oxalate is directly advantageous to the thallus by virtue of the 

 capacity of the crystals to reduce or prevent evaporation, as has been 

 pointed out by Zukal 1 . A like service afforded by crystals to the leaves of 

 the higher plants in desert lands has been described by Kerner 2 . These 

 are frequently encrusted with lime crystals 'which allow the copious night 

 dews to soak underneath them to the underlying cells, while during the day 

 they impede, if they do not altogether check, evaporation. 



Calcium oxalate crystals are insoluble in acetic acid, soluble in hydro- 

 chloric acid without evolution of gas; they deposit gypsum crystals in 

 a solution of sulphuric acid. 



C. OIL-CELLS 



a. OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens 

 are able to dissolve the lime of the substratum, and their hyphae penetrate 

 more or less deeply into the rock. In some forms the entire thallus may 

 thus be immersed, the fruits alone being visible on the surface of the stone. 

 In two such species, Verrucaria calciseda and Petractis (Gyalecta) exanthe- 

 matica, Steiner 3 detected peculiar sphaeroid or barrel-shaped cells that 

 differed from the other hyphal cells of the thallus, not only in their form, 

 but in their greenish-coloured contents. Similar cells were found by Zukal 4 

 in another immersed (endolithic) lichen, Verrucaria rupestris f. rosea. He 

 describes them as roundish organs crowded on the hyphae and rilled with a 

 greenish shimmering protoplasm. He 5 found the same types of sphaeroid 

 and other swollen cells in the immersed thallus of several calcicolous lichens 

 and he finally determined the contents as fat in the form of oil. He found 

 also that these fat-cells, though very frequent, were not constantly present 

 even in the same species. His observations were confirmed by Hulth 6 for 

 a number of allied crustaceous lichens that grow not only on limestone but 

 on volcanic rocks. In them he found a like variety of fat-cells intercalary or 

 torulose cells, terminal sphaeroid cells and hyphae containing scattered oil- 

 drops. Bachmann 7 followed with a study of the thallus of purely calcicolous 

 lichens. The specialized oil-cells were fairly constant in the species he 

 examined, and, as a rule, they were formed either in the tissues immediately 

 below, or at some distance from, the gonidial zone. Funfstiick 8 has also 



1 Zukal 1895, p. 1311. 2 Kerner and Oliver 1894, p. 235. 3 Steiner 1881. 4 Zukal 1884. 

 6 Zukal 1886. 6 Hulth 1891. 7 Bachmann 1892. 8 FunfstUck 1895. 



