604 The Outline of Science 



of some of the simplest, they depend for their racial survival on 

 vegetative increase, on endowing their offspring with large 

 legacies, and on being able to lie low within retrenchments abun- 

 dantly provided with stores. 



The Essential Parts of a Flowering Plant 



The poet Goethe was one of the first to see clearly that an 

 ordinary flowering plant consists essentially of two parts the 

 axis and the appendages. The axis includes (1) the upward- 

 striving stem, which seeks the light and grows against gravity, and 

 (2) the downward -boring root, which avoids the light and grows 

 towards the centre of the earth. The appendages are the leaves 

 which the stem spreads out to the light and in the air; and the 

 flower consists typically of four tiers, or whorls, of transformed 

 leaves the sepals, petals, stamens, and carpels the two last 

 producing reproductive cells. It is interesting to examine in the 

 spring the opening buds on the horse-chestnut tree, for they show 

 so clearly every grade between the scales that protect the bud and , 

 the ordinary leaves with their five leaflets. And a little later we 

 may profitably examine the beautiful flower of the white water- 

 lily to see the transitions between the green sepals and the white 

 petals, and the transitions between the petals and the pollen-pro- 

 ducing stamens. When a wild flower like a briar rose goes 

 "double" in a garden, this means that a large number of trans- 

 formed leaf -structures, which should have become stamens, have 

 gone back into sterile petals. We see, then, that the general 

 architecture of a plant is very simple compared with that of most 

 animals. 



1 



The Laboratory of the Green Leaf 



It is now necessary to consider in more detail how plants feed. 

 Plants receive their raw material by means of their leaves and by 

 their roots. The green leaves of plants receive carbonic acid gas 



