246 LICHENS. 



have fallen down into the valley beneath, one can only explain the phenomenon by 

 supposing that the algal and fungal cells concerned have been blown together, and 

 that the opportunity has been afforded them on the blocks of stone of contracting 

 a union. Now, so far as regards one of the two partners, viz.: the one devoid of 

 chlorophyll, and known as a fungus — the idea that everywhere in the air spores of 

 fungi are swarming about is so familiar to us that the supposition of an occasional 

 stranding of individual spores, which are being blown about by the wind, upon the 

 moist broken surfaces of stones can encounter no opposition. Respecting those 

 spores in particular which are ejected from the aerial fructifications of lichens, the 

 discussion of their life-history and distribution must of course be reserved for a 

 later section; but it is necessary to make here the one statement that provision 

 exists for the most profuse and distant dissemination of these spores. 



Thus, in the case of one of the partners, there is no difficulty in realizing its 

 ubiquity. But when one comes to the Algae, the name at first calls up to mind the 

 green filaments which occupy our pools and ponds, or the brown wracks and red 

 Florideas of the sea-shore, and we ask ourselves how it can be possible for these 

 plants to occur on fractured surfaces of stone, especially on the debris of mountain 

 sides. Indeed, it is certainly not Algse of these kinds that take part in the 

 construction of Lichens. The name Algae is properly only a general name for all 

 Thallophytes containing chlorophyll, and it is applied to many small organisms 

 besides those mentioned above, namely, to numbers of Nostocineae, Scytonemeae, 

 Palmellaceae, Chroolepideae, and these are the kinds which fall in with the cells of 

 fungi and form lichens in conjunction with them. Owing to their minute size, 

 they are apt to escape observation, and, in general, only attract attention when 

 myriads of them clothe the bark of trees, cliffs, stones, or earth. In these situations 

 they need but little moisture, and it is not necessary for any of them to live under 

 water like other algae^; they become desiccated without sustaining the slightest 

 injury and make their appearance on the substratum occupied by them at the first 

 stage of their development, as powdery coats, and, in this condition being extremely 

 light, are liable to be blown away by a wind of moderate strength, and so 

 distributed over mountain and valley. 



That this dissemination is not merely hypothetical but an actual fact has been 

 susceptible of easy proof by the following experiment, made in a mountain-valley in 

 the Tyrol. A plane surface covered with white filter-paper, which was kept moist, 

 was exposed to a south wind; in the course of a few hours numerous particles, like 

 dust, adhered to the paper, and amongst them cell-groups of Nostocineae and others 

 of the above-mentioned algae occurred regularl}?-, in addition to organic fragments 

 of the most various kinds, such as pollen-grains and spores of all sorts of mosses 

 and fungi. All these bodies were deposited in the little depressions on the sheet 

 of paper, and in the same way they rest in the grooves, cavities, and cracks in the 

 surfaces of stone, bark, and old wood-work, where they succeed in reaching a 

 further development as soon as the requisite quantity of water is provided. Now, 

 if at these places the little algal cell-groups meet with hyphae belonging to the 



