598 COTYLEDONS. 



COTYLEDONS. 



The cotyledons or seed-leaves are borne by the embryonic stem. Their 

 function in the first instance is to provide this organ, as well as the rudiments 

 of the radicle at the one end and the small bud at the other, with food. These 

 portions of the embryo as long as they are still surrounded by the skin-like 

 envelope of the seed — the so-called " seed-coat " — and even still later, when they 

 have burst through these envelopes, cannot at once absorb inorganic food from 

 their environment, and still less can they transform this into organic materials. 

 And yet they require these substances for growth, that is to say, they require 

 materials for the building of the foundation of the plant-body which is to issue 

 from the seed. Only when the radicle has penetrated into the soil and 

 produced its root-hairs, and green leaves have forced their way to the sunlight 

 from the little bud which formed the rudiment of the epicotyl, is the young, 

 newly-settled plant placed on its own feet ; henceforth it can nourish itself 

 independently. But up to the moment of this independence it draws its food 

 from a store which is deposited in the seed ; it lives on materials derived from the 

 mother-plant, i.e. on a supply of starch, fat, and proteid formed by the parent 

 and deposited in special cells for the benefit of the embryo. A fully-equipped 

 embryo is provided with food reservoirs in either of two ways. Sometimes 

 the cotyledons themselves form the storehouse for the food to be consumed 

 later on. In this case the reserve materials are deposited by the parent plant 

 in the cell-chambers of the cotyledon, and when the suitable time arrives, and 

 when the need for them has arisen, these materials are employed in the 

 further construction of the hypocotyl, and of the radicle, springing from one 

 end of it, and of the bud at the other end. In the second case a special store- 

 house is formed within the enveloping seed-coat beside the embryo. The cells 

 of this storehouse are quite filled with fat and starch and proteid granules. 

 The tissue of this particular store-chamber of the embryo is in most instances 

 composed of cells which have arisen, together with the germ-cell, in the so-called 

 embryo-sac (the large cell in which the egg is produced), and it is then termed 

 endosperm. Less frequently this tissue is formed outside the embryo -sac, in 

 the nucellus, and is then called perisperm. This distinction is without signifi- 

 cance in the processes here to be discussed, and therefore in the following 

 description, endosperm and perisperm are included under the term reserve- 

 tissue. 



When the cotyledons themselves form the reserve -tissue, the maintenance 

 of the young plant is relatively simple. The transformation and transportation 

 of the reserve-materials are carried on in the manner already described. In 

 proportion as the radicle of the embryo develops into the root, and a leafy 

 shoot is produced from the embryonic bud at the cost of the building materials 

 conveyed to them, the cells of the cotyledons lose their store of food materials, 



