DISPERSAL BY WATER. 811 



white as a crushed grain of corn. As they get older the crusts become rent, and 

 separate either partially or wholly from their substratum. When they first become 

 loosened the edges of the detached portion become somewhat rolled back. The roll- 

 ing then continues, and ultimately the loosened piece forms an elliptical or spherical 

 warted body with a very much contracted central cavity. Small stones are some- 

 times imprisoned in this way within the cavity of the sphere, in which case the 

 weight of the loose Lichen is correspondingly increased, but as a rule the hole is 

 filled with air, and when dried the pieces weigh very little. Ten loose pieces of 

 Manna-lichen, each as large as a hazel-nut, weighed 3-36 grams, and the weight of 

 a single piece therefore was on an average only -34 grams. It is easy to see that 

 the loose portions will be rolled about by the wind, and that a storm will sometimes 

 sweep them up from the ground and carry them hither and thither through the air. 

 This method of distribution appears to be the prevailing one in regions where the 

 supply of water is not abundant in the rainy season, and where violent storms rage 

 from time to time. That this is so is confirmed by the circumstance that the Manna- 

 lichen after the storms lies chiefly piled up behind the low bushes and undergrowth,. 

 i.e. just where the force of the storm has been to some extent broken, and where the 

 shifting sand has been heaped up into little hillocks. Where a period of heavy 

 rains succeeds the long dry summer, however, and where such a quantity of water 

 falls on the parched land that it cannot all be absorbed, some of the rain collects 

 into small rivulets. These carry away with them everything which is movable 

 and capable of floating. The turbid rivulets flow down over the inclined soil to the 

 lowest parts of the country and there unite into larger streams, or if it can find no 

 outlet the water remains for some time in the hollows as small pools and puddles, 

 and deposits there the mud and vegetable debris it has carried with it. The latter 

 is more especially the case on the steppe soil overstrewn with small stones where 

 between the slight elevations there is a labyrinth of shallow channels and winding 

 depressions resembling ploughed land. In such regions the Manna-lichen is chiefly 

 washed into the depressions by the rain-water, and in some years in such quantity 

 that they form heaps a span high, and a single man can in a day collect 4-6 

 kilograms (about 12,000-20,000 pieces, varying in size from a pea to a hazel-nut). 

 This is especially the case in the steppe region and in the high lands of South-west 

 Asia, where the Manna-lichen is used as a substitute for corn in years of famine — 

 being ground in the same way and baked into a species of bread. That the rain- 

 water is the agent which transports the Lichen in these regions is beyond doubt,, 

 because the pieces heaped up in the hollows are not in the least rubbed on their 

 outer surfaces as would certainly be the case if they had been rolled and dragged 

 even for only a short distance over stony ground. It is also remarkable that 

 all the great so-called rains of manna, of which news has come from the East 

 to Europe, especially those of the years 1824, 1828, 1841, 1846, 1863, and 1864, 

 occurred at the beginning of the year between January and March, i.e. at the time 

 of the heaviest rains. When we remember that the inhabitants of the district 

 actually thought that the manna had fallen from heaven, and quite overlooked the 



