More or less specialized oil-cells have been demonstrated by Fünfstück[801] in several superficial (epilithic) lichens which grow on a calcareous substratum, as for instance Lecanora (Placodium) decipiens, Lecanora crassa and other similar species. The oil in these lichens is usually restricted to more or less swollen or globose cells; but it may also be present in the ordinary hyphae as globules. Zukal[802] found that the smooth little round granules sprinkled over the thallus of the soil-lichens, Baeomyces roseus and B. rufus, contained in the hyphae typical sphaeroid oil-cells and that they were specially well developed in specimens from Alpine situations. In still another soil-lichen, Lecidea granulosa, shimmering green oil was found in short-celled torulose hyphae.
Rosendahl’s[803] researches on the brown Parmeliae resulted in the unexpected 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[804] 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. Fünfstück[805], on the other hand, considers the oil of the sphaeroid and swollen cells as an excretion, representing the waste products of metabolism 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 rarely the case, the maximum of fat production being always at some distance.
Fünfstück 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[806] also placed in the dark two quick-growing calcicolous lichens, Verrucaria calciseda and Opegrapha saxicola. At the end of the experiment, he found that they had increased in size without using up the fat. Lang[807] 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 Fünfstück that the oil is an excretion and represents a waste-product of the plant.
Considerable light is thrown on the subject of oil-formation by the results of recent researches on the nutrition of algae and fungi. Beijerinck[808] made comparative cultures of diatoms taken from the soil, and he found that so long as culture conditions were favourable, any fat that might be formed was at once assimilated. If, however, some adverse influence checked the growth of the organism while carbonic acid assimilation was in full vigour, fat was at once accumulated. The adverse influence in this case was the lack of nitrogen, and Beijerinck considers it an almost universal rule in plants and animals, that where there is absence of nitrogen, in a culture otherwise suitable, fat-oils will be massed in those cells which are capable of forming oil. He observed that in two of the cultures of diatoms the one which alone was supplied with nitrogen grew normally, while the other, deprived of nitrogen, formed quantities of oil-drops. Wehmer[809] records the same experience in his cultural study of Aspergillus. Sphaeroid fat-cells, similar to those described by Zukal in calcicolous lichens, were formed in the hyphae of a culture containing an overplus of calcium carbonate, and he judged, entirely on morphological grounds, that these were not of the nature of reserve-storage cells.
Stahel[810] has definitely established the same results in cultures of other filamentous fungi. In an artificial culture medium in which nitrogen was almost wholly absent, the cells of the mycelium seemed to be entirely occupied by oil-drops, and this fatty condition he considered to be a symptom of degeneration due to the lack of nitrogen. These experiments enable us to understand how the hyphae of calcicolous lichens, buried deep in the substratum, deprived of nitrogen and overweighted with carbonic acid, may suffer from fatty degeneration as shown by the formation of “sphaeroid-cells.” The connection between cause and effect is more obscure in the case of lichens growing on the surface of the soil, such as Baeomyces roseus, or of tree lichens such as the brown Parmeliae, but the same influence—lack of sufficient nitrogenous food—may be at work in those as well as in the endolithic species, though to a less marked extent.
It seems probable that the capacity to form oil- or fat-cells has become part of the inherited development of certain lichen species and persists through changes of habitat as exemplified in Lecanora calcarea[811].
In considering the question of the formation and the function of fat in plant cells, it must be remembered that the service rendered to the life of the organism by this substance is a very variable one. In the higher plants (in seeds, etc.) fat undoubtedly functions in the same way as starch and other carbohydrates as a reserve food. It is evidently not so in lichens, and in one of his early researches, Pfeffer[812] proved that similarly oil was only an excretion in the cells of hepatics. He grew various species in which oil-cells occurred in the dark and then tested the cell contents. He found that after three months of conditions in which the formation of new carbohydrates was excluded, the oil in the cells, instead of having served as reserve material, was entirely unchanged and must in that instance be regarded as an excretion.