12 FLOWERLESS PLANTS 



slime is present. This is composed of masses 

 of algce, myriads of which are always floating 

 about in the air ready to settle on any moist 

 position. The growth of the algse all tends 

 towards the break up of the hard and un- 

 inviting surface of the wall, and quite soon 

 mosses and lichens appear upon the scene. 

 Probably the mortar will become outlined with 

 velvety green moss, whilst big patches of grey 

 lichens will spread themselves over the surface 

 of the bricks. These humble plants are able 

 to secure all they need from the air and water, 

 coupled with the small amount of mineral 

 matter which they can absorb from the material 

 of which the wall is constructed. Generations 

 of mosses live and die upon the wall, and 

 eventually in the cracks and crevices there 

 will be quite a nice accumulation of decayed 

 vegetable matter. A gust of wind, or a passing 

 bird, is responsible for the placing of the seed 

 of the first flowering plant to grow up on this 

 world in miniature. Years go by, and as this 

 wall is still left in the hands of Nature, the 

 accumulation of soil from decayed plants may 

 be so considerable that even a small tree is 

 able to establish itself. Thus within the span 



