338 HOW PLANTS LIVE AND WORK. 



edge of the leaf, and the edge is turned over so as to cover them 

 in. Here the scales are unnecessary, and are therefore not 

 formed. 



When the spores are ripe, each box becomes dry, and is ulti- 

 mately burst by the sudden straightening of a spring which is 

 coiled round its edge. The force of the uncoiling of the spring is 

 sufficient to jerk the spores out of the box into the air, and they 

 may be carried for some distance by the wind before they at 

 length reach the ground. Once there, however, each spore, under 

 favourable conditions, begins to grow, and gives rise to a plant, 

 which, curiously enough, is not in the least like the parent Fern 

 plant which produced the spore. The new plant rejoices in the 

 name of Prothallus. It is important to notice that the spore is 

 produced by a purely non-sexual process. In this respect it differs 

 widely from a seed, which, it will be remembered, results from the 

 union of the protoplasm of a pollen grain with that of an ovule. 

 The Prothallus is a flat, filmy little plant of the form which is 

 generally called heart-shaped. It has neither stem nor roots, but, 

 as its cells contain chlorophyll, and as it puts out on the lower 

 surface little hairs which take up watery solutions from the soil, it 

 is in no danger of starvation, and is quite capable of taking care 

 of itself and leading an independent existence. This tiny plant 

 (the biggest Fern Prothallus I have seen was about the size of a 

 3d. piece), in contrast with its parent, produces sexual organs. 

 Some of these organs give rise to male cells and others to female 

 cells. The male cells are excessively small, and can only be seen 

 by the high power of a microscope. When they are ripe they 

 swim about in a drop of dew or rain, just like little animals, and 

 find their way to the female cells, which they fertilise. The 

 embryo which results from the union grows up into an ordinary 

 Fern plant. There is thus in Ferns a very sharply marked alterna- 

 tion of generations. The first generation, the ordinary Fern, 

 which is non-sexual, produces, by means of spores, the sexual 

 generation, called a Prothallus. The Prothallus gives rise, in its 

 turn, by a sexual process, to the obvious Fern plant. Each genera- 

 tion, therefore, resembles, not its parent, but its grandparent. 



The Horsetails, sadly reduced though they are, and, at least in 

 British species, showing but little evidence of the former glory of 



