THE LIFE AND GROWTH OF SEEDLINGS 9 



In his work on the Movements of Plants, Darwin gives some 

 interesting experiments which he devised in order to form some 

 idea of the upward growth of the plumule after it had straightened 

 itself. He found in the case of the common bean, that " the basal 

 leg grew upwards with a force sufficient to lift a thin plate of zinc 

 loaded with 12 ounces. Two more ounces were added, and the 14 

 ounces were lifted up to a very little height, and then the epi- 

 cotyl yielded and bent to one side " (Movements of Plants, p. 88). 



HYPOGEAL COTYLEDONS. The cotyledons of some plants 

 never leave the seed, partly because the testa does not readily 

 split, and it is therefore difficult to extricate the seed-leaves. 

 Those of the pea, the broad bean, and the kidney bean do not 

 (although those of the dwarf French Bean and other species do) ; 

 neither do those of the oak, the horse chestnut, and the walnut. 

 In the month of May in an oak wood, numbers of acorns will be 

 seen on the ground sprouting, with the cotyledons inside the nut ; 

 whilst in the beech, the holly, the gorse, and in the majority of 

 trees and shrubs, the cotyledons will be found above ground. 



As a rule, the cotyledons which do not come above ground are 

 thick and fleshy, for they have absorbed all the food-material 

 formed in the ovule after fertilisation. These are exalbuminous 

 seeds. So too are the seeds of the radish, mustard, cucumber ; 

 but these have smaller and more delicate seed-leaves, which come 

 above ground, turn green, and are able to take in food-material 

 from the air and make food for the young plant. 



The wheat, maize, barley form another group. They are 

 albuminous seeds, i.e. they contain food-material apart from the 

 embryo, in that part of the seed known as the endosperm. 



ALBUMINOUS SEEDS COMPARED WITH EXALBUMINOUS. To 

 realise the difference in the structure of a grain of wheat and the 

 seed of a pea, soak a few peas and grains of wheat for twenty-four 

 hours and then examine them. The seed-coat of the pea is 

 tightly stretched, owing to the water absorbed, the outlines of the 

 triangular radicle can be seen, the apex pointing towards the 

 scar. On removing the seed-coat, the whole of the seed is seen to 

 be occupied by the two cotyledons with the plumule between them, 



