ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 353 



Lichens. 



(By A, LoRRAiN Smith.) 



Rhizoids of Granite-inhabiting Lichens.* — E. Bachinan remarks 

 on the extreme difficulty of examining such delicate structures as 

 rhizoids and hyphre in so unyielding a substance as granite. He found 

 that the easiest way to attack the problem was to examine the mica 

 crystals. When these crystals are parallel with the surface the hyphas 

 are spread over them. When the crystals are at right angles to the 

 surface or in a slanting position, it is found that they are pierced and 

 split into many folds by the penetrating rhizoids and gonidia. In time, 

 owing to the action of the lichen, the mica loses its shining appearance, 

 and becomes white like chalk. This change has been traced to a depth 

 of 4 mm., but it varies with the nature of the granite and with the 

 lichen species. It seems to be proved that chemical action by the 

 hyphae on the crystals results in the formation of minute openings and 

 channels in the mica ; these become filled with water and afford a satis- 

 factory condition for further growth and penetration of the hyphas. 

 This results in the splitting apart and the disintegration of the crystal, 

 and a subsequent still more' luxuriant development of the plant. In 

 comparison with chalk-inhabiting lichens, it is found that, while the 

 latter have a well-developed independent gonidial zone in the sub- 

 stratum, in the granite there is, at most, only an occasional layer of 

 green cells, and these always connected with the surface layer. The 

 rhizoidal portion of the lichens is formed of three principalco n- 

 stituents : (1) Delicate colourless hyphse with long cells, richly branched 

 and often anastomosing ; (2) short-celled, thick-walled, green, brownish- 

 green, or brown hyphse, sometimes necklace-like, connected with the 

 protothallus ; and (o) globose cells which have been found in almost all 

 the lichens examined. They are filled with oil, and are comparable with 

 the oil-containing cells of other lichens, especially of those that are found 

 on chalk or limestone. In the case of the latter the thallus is almost 

 entirely imbedded in the rock, and does not differ in structure from the 

 external layers. The author has studied the effect of the rhizoids on 

 other granite crystals, though not so fully, and comes to the general 

 conclusion that splitting ot the crystal is the method by which the 

 lichen penetrates the substratum. These loosened areas are found to be 

 filled with a mass of hyphse and gonidia. 



Bachmann closes the paper by a detailed account of the different 

 lichens he examined, and the points to be noted in connection with each 

 species. These were : Sphyridiimi byssoides, Aspicilia gibbosa, Rhizo- 

 carpon geographicum, Acarospora discreta, Pertusaria corallina, and 

 Rhizocarpon atroalbum, all of them crustaceous forms. 



Symbiosis in Lichens.t — A. Elenkin has written a paper on the 

 conception of symbiosis as a " movable equilibrium of the symbionts." 

 If, he says, we could conceive a state where the conditions of life 



* Jahr. wiss. Bot., xliv. (1907) pp. 1-40 (2 pis.). 



t Bull. Jard. Bot. St. Petersbourg, xi. (1906) pp. 1-19. See also Bot. Centralbl., 

 civ. (1907) pp. 175-6. 



