CELLS AND CELL PRODUCTS 219 



pected discovery of specialized oil-cells situated in the cortices — upper and 

 lower — of five species out of fourteen which he examined. In one of the 

 species, P. papulosa, they also occurred in the cortex of the rhizoids. The 

 oil-cells were thinner-walled and larger than the neighbouring cortical cells; 

 they were clavate or ovate in form and sometimes formed irregular external 

 processes.^ They were more or less completely filled with oil which coloured 

 brown with osmic acid, left a fat stain on paper and, when extracted, burned 

 with a shining reddish flame. These oil-cells were never formed in the 

 medulla nor in the gonidial region. 



c. Significance of Oil-formation. Zukal^ regarded the oil stored 

 in these specialized cells as a reserve product of service to the plant in the 

 strain of -fruit-formation, or in times of prolonged drought or deprivation of 

 light. According to his observations fat was most freely formed in lichens 

 when periods of luxuriant growth alternated with periods of starvation. He 

 cites, as proof of his view, the frequent presence of empty sphaeroid cells, 

 and the varying production of oil affected by the condition, habitat, etc. of 

 the plant. Fiinfstuck^, on the other hand, considers the oil of the sphaeroid 

 and swollen cells as an excretion, representing the waste products of meta- 

 bolism in the active tissue, but due chiefly to the presence of an excess of 

 carbonic acid which, being set free by the action of the lichen acids on the 

 carbonate of lime, forms the basis of fat-formation. He points out that the 

 development of fat-cells is always greater in endolithic species in which the 

 gonidial layer — the assimilating tissue — is extremely reduced. In epilithic 

 lichens with a wide gonidial zone, the formation of oil is insignificant. He 

 states further that if the oil were a direct product of assimilation, the cells 

 in which it is stored would be found in contact with the gonidia, and that 

 is rarel)' the case, the maximum of fat production being always at some 

 distance. 



Fijnfstiick tested the correctness of his views by a prolonged series of 

 growth experiments; he removed the gonidial layer in an endolithic lichen, 

 and found that fat storage continued for some time afterwards, its production 

 being apparently independent of assimilative activity. The correctness of 

 his deductions was further proved by observations on lichens from glacier 

 stones. In such unfavourable conditions the gonidia were scanty or absent, 

 having died off, but the hyphae persisted and formed oil. He' also placed 

 in the dark two quick-growing calcicolous lichens, Verrncaria calciseda and 

 Opegrapha saxicola. At the end of the experiment, he found that they had 

 increased in size without using up the fat. Lang* also is inclined to reject 

 Zukal's theory, seeing that the fat is formed at a distance from the tissues 

 — reproductive and others — in need of food supply. He agrees with Fiinf- 

 stiick that the oil is an excretion and represents a waste-product of the plant. 



1 Zukal 1895. ^ Funfstuck 1896. " Fimfstiick 1899. * Lang 1906. 



