ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 291 



Origin of Lichen Gonidia.* — An old controversy as to the nature 

 and origin of the green cells within the lichen thallus, has been revived 

 by Fried. Elfving. He has made examinations of the growing areas of 

 a number of lichens, and claims to have observed the different stages of 

 formation of gonidia in such widely diverse " algal " forms as Cystococcus, 

 Trmiepohlia, Stiffonema, and Nostoc. According to the view put forward 

 by Eifving, the gonidium arises terminally as regards Cystococcus, inter- 

 calary as in Trentepohlia on the colourless filaments of the lichen fungus. 

 There is first a swelling of the destined gonidial cell, and gradually a 

 colouring and transformation of the contents to the green assimilating 

 function. A research was also made on the formation of cephalodia. 

 These arise, according to Elfving, by a branching and mingling of the 

 external hairs of the thallus (in Peltidia aphthosa), and after the forma- 

 tion of a ball of tissue the differentiation begins of the central cells to 

 Nosioc and subsequent growth to Nostoc chains. 



Elfving demands a review of the theory of the dual nature of the 

 lichen thallus, which, after the observations made by him, is no longer 

 tenable. He allows that the gonidia mainly increase by division within 

 the thallus, and that they also may escape and live as free organisms, 

 and that thus " certain algae are derivatives from lichen gonidia." 



The work is richly provided with illustrations and with a copious 

 bibliography. 



Lichen Ecology. f — The results of some preliminary researches on 

 the general ecology of lichens have been published by 0. Galloe. The 

 two factors more especially studied are climate and substratum. The 

 former is not of great importance. As concerns the latter he finds that 

 lichens fall into six groups : viz. (1) bark lichens ; (2) epiphyllous 

 lichens : earth lichens ; (4) rock lichens ; (5) parasitic, and (6) sapro- 

 phytic lichens. Each of these groups include crustaceous, foliose, and 

 fructicose forms. Certain lichens are always found on the same kind of 

 bark, but the reasons therefor are not sufficiently known. Epiphyllous 

 lichens never grow on annual leaves ; mostly, they are found on tropical 

 plants. Earth lichens are peculiarly dependent on the nature of the sub- 

 stratum ; lichens are never found on light humus, turf, or salt soil. The 

 water content of the soil is also of great importance, more so than the 

 condition of the temperature. The amount of nourishment in the soil 

 is of little influence ; but as a rule lichens are ousted from good soil 

 by the pressure of the higher plants. The chemical reaction is of some 

 significance, and most species grow willingly on acid soils. In rock 

 lichens the character of the surface is of importance, as also the presence 

 of lime. 



In a separate chapter, the author discusses the biology of various 

 fruticose lichens, such as Gladonia, Stereocaulon, Alectoria, Bryopogon, 

 etc. 



* Acta Soc. Sci. Fenn., xliv. No. 2 (1913) 71 pp. (8 pis.). 



t Forbered. Unders. til en Alrnind. Lykenologi. Kjobenhavn : (1913) 118 pp. 

 240 figs. See also Bot. Centralbl., cxxv. (1914) pp. 169-70. 



