810 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



important part in the covering of tree-trunks with Mosses and Liverworts. A 

 small patch of Radula, Metzgeria or similar plant having once taken hold, when 

 a downpour of rain beats upon the trunk quantities of tiny ball- and disc-like 

 offshoots float away to be caught again by projecting irregularities of the surface; 

 indeed the rapid covering of old trunks with green carpets and mantles of 

 Liverworts and Mosses is for the most part effected by rain-water. 



It is comparatively seldom that bud- and sprout-shaped offshoots are distri- 

 buted by rain. But there is one very interesting example of this, viz. the widely- 

 spread Lesser Celandine (Ranunculus Ficaria), a single plant of which is shown in 

 fig. 343 3 , p. 460. In the axils of the foliage-leaves of this plant are developed off- 

 shoots which have the form of small tubers, and which are not unlike the youngest 

 stages of small potato-tubers (fig. 343 6 ). When the leaves and stalk of the Lesser 

 Celandine begin to turn yellow and wither in the early summer, the tubers break 

 away from the stem and fall to the ground. There they usually escape observation, 

 since they are hidden by the yellowing foliage; but should there come a heavy 

 storm of rain the withered leaves are pressed down on to the soil by the force of the 

 rain-drops, and the scattered tubers become visible. Sometimes the impact of the 

 falling rain-drops hastens the detachment of the tubers from the mother-plant. 

 When the rain is so heavy that the water flows away in the form of small rivulets, 

 the loose tubers are washed off in abundance. A sudden downpour of rain in a 

 region abundantly overgrown with Lesser Celandine is sufficient to float away 

 numbers of the tubers, and heap them up on the borders of irrigation channels 

 when the rain disperses. In such places the quantity of tubers which have floated 

 together is often so large that one can hardly gather them in one's hands. In this 

 way arose the idea that the tubers had fallen from heaven with the rain, and the 

 myth of a rain of potatoes. 



The small tubers which arise in the axils of the leaves of Gagea bulbifera 

 (cf. fig. 343 \ p. 460), a plant growing on the steppes of Southern Russia, are distri- 

 buted by rain-water just like those of the Lesser Celandine. This brings us to the 

 question of the much-discussed manna-rain in steppes and deserts, which in reality 

 is nothing but the distribution of the offshoots of a Lichen, viz. the Manna-lichen. 

 This Lichen, which was termed Lichen esculentus by the older Botanists, but in 

 recent times has been referred to the genera Urceolaria, Lecanora, Chlorangium, 

 and Sphcerothallia, and which apparently consists of three species, viz. Lecanora 

 esculenta, L. desertorum, and L. Jussufii, is spread over an enormous region in 

 south-west Asia, and extends as far as the south-east of Europe and the north of 

 Africa. This Lichen is met with in the neighbourhood of Constantinople, in the 

 Crimea and Caucasus, in Persia (whence the illustration at page 695), also in 

 Kurdistan, Arabia, and the Anatolian high land from Bulgar Dagh in the Taurus 

 (where it is very often met with at a height of 2700 metres above the sea), and 



illy in the Sahara and the deserts of Algeria. It first forms thick wrinkled and 



d crusts on the stones, preferably on small fragments of limestone lying about; 



outer colour of the crust is a grayish yellow, while on breaking it appears as 



