638 The Outline of Science 



the egg-cell, which it fertilises. The fertilised egg-cell develops 

 into a new fern plant. A single cycle of the life-history of the 

 fern is divided into two phases, a sexual prothallus and an asexual 

 spore-bearing fern plant. There are two alternating generations. 

 The mosses, a side branch of the evolutionary tree, show the same 

 features. The moss plant is the sexual generation; the capsule, 

 parasitic on its parent, is the spore-bearer. The resting distribut- 

 ing body is, in both mosses and ferns, the spore. 



11 



The main line of evolution was that which led through the 

 fern-like plants to the flowering plants. There was evolved a 

 vegetative equipment suitable to life in moist soil and dry air; 

 absorbing root-hairs, roots, water-conducting tissues; light- 

 absorbing leaves, borne aloft on a branching stem. The method 

 of reproduction was gradually transformed till, in the flowering 

 plant, a stage has been reached where the original equipment is 

 barely recognisable. The plant is definitely emancipated from 

 the medium in which it took its origin; fertilisation is now pre- 

 ceded by pollination, and in providing for that the plant has 

 evolved, in relation to the insect, the flowers we know to-day. 



The Fall of the Leaf 



Some plants are annuals and some perennials. Once annuals 

 have flowered and the seed has been set, they wither and die. 

 Perennials live on for several seasons. In these, and in shrubs 

 and trees, reserve food stores of starch and oil are available in 

 spring for forming the new foliage. Lilies and onions store 

 reserve material, accumulated during the summer, in their bulbs. 

 Other plants store material in underground tubers, which may 

 be modified stems or roots. We may fitly close our study of 

 the living plant by picturing the fall of the leaf, which so often 

 marks the end of the year's activity. We have taken the follow- 



