140 THE BOOK OF NATURE STUDY 



absorb light and at once begin to make food. A heavy fall 

 of rain after long drought also enables the plant to produce its 

 spore fruits, which can often be obtained simply by moistening 

 a, lichen which has remained dry for some time. A few lichens 

 swell up and become slimy or jelly-like when moist, thus retain- 

 ing water for a long time, but most lichens have not this power 

 and soon dry up again after having absorbed water, especially 

 when exposed to sun and wind. Like other evergreen plants 

 subject to extremes of temperature and the ever-present danger 

 of desiccation, lichens are tough and resistant, and very few of 

 them can thrive in the moist places which suit other plants. 



It is interesting to compare the " bark flora " on different 

 parts of a tree. As every observant person knows, a tree, when 

 not growing in a crowded wood, has a distinct rain-side exposed 

 to the prevailing wet wind, and the wettest part of the tree is 

 on the rain-side at the base of the trunk, where rain-water trickles 

 downwards. This is especially marked in rough-barked trees, 

 like oaks. On the rain-side we find chiefly mosses and liver- 

 worts, with here and there a tufted or foliose lichen, while the 

 drier opposite side has few mosses and is often entirely covered 

 with crust-lichens. Higher up the trunk the bark becomes drier, 

 partly because rain-water runs off and partly because of the 

 stronger wind which causes drying, so that the conditions become 

 more favourable to lichens, which here grow all round the trunk. 

 In many cases, too, one can observe a fairly regular zonal arrange- 

 ment of the different lichen species at different heights on a trunk. 



On trees with smooth bark, like beeches, very little water 

 collects and very few mosses grow, though some liverworts (e.g. 

 Fmllania, Metzgeria, Radula) grow in profusion, but this dry 

 substratum suits Graphis and the other crust-lichens which often 

 entirely cover the bark. 



