148 METABOLISM 



may consider, finally, translocation products. It must be noted, however, that 

 this subdivision, based on the function of these substances in the plant, gives 

 no indication whatever of the chemical nature of the compounds. The four 

 different types of material may be chemically distinct but they need not ; 

 glucose, for example, may occur as a primary assimilation product, as a trans- 

 location compound, as a plastic substance, a reserve, or as a source of energy. 



It would be obviously most natural to commence our study of the migra- 

 tion and translocation of the products of assimilation by observation of the 

 method by which such substances are removed from the leaf ; but for many 

 reasons it is advisable to begin with the reserves, which are usually regarded as 

 assimilatory products redeposited in ' secondary storehouses '. Reserves are 

 deposited in these storehouses in such quantities that plants are often able at 

 their expense, to develop to a considerable extent in darkness without needing 

 to have recourse to any direct products of assimilation. Leaves, on the con- 

 trary, contain very little in the way of reserves, are rapidly deprived of them, 

 and are liable to injury if kept in darkness for any length of time. Conse- 

 quently the most important researches have been carried out on storehouses 

 of reserves, and more especially on seeds. When we have mastered the con- 

 ditions which prevail in these structures the transformations which take place 

 in the foliage leaves will be easily understood. 



The most important constituent of every seed is the embryo. It consists of 

 a small, often microscopic young plant, in which we may distinguish one or two 

 more or less well developed cotyledons, all other parts being still in an embryonic 

 condition. Between the cotyledons may be distinguished the plumule, or growing 

 point of the stem, surrounded by a few leaves, and, at the other end, the 

 growing point of the root, the radicle. The whole embryo is in general enclosed 

 in a special tissue, the endosperm, and that in turn by a seed-coat. When the 

 seed is separated from the parent it cannot at once undergo development, for it 

 is deficient in moisture, without which growth is impossible. In addition to 

 certain other external factors which are conditions of germination, viz. warmth 

 and oxygen, water is primarily essential ; when that is supplied the seedling 

 begins to grow. Generally speaking, the root bursts the seed-coat and imbeds 

 itself in the soil ; later on, the plumule is extended and gives rise to leaves above 

 ground. As soon as the leaves develop a green colour on exposure to light, the 

 plant becomes independent and can nourish itself by products which it itself 

 has manufactured, but its entire development up to this stage is possible only if it 

 be provided with reserves supplied to it by the parent. These reserves are as 

 a rule capable of supplying all that is necessary for much later stages in the 

 development of the seedling, so that from large seeds, such as those of the bean, 

 plants of considerable dimensions may be produced in the dark, entirely at the 

 cost of these reserves. The reserves are often deposited in the seedling itself, 

 and the relatively bulky seed-leaves are frequently the seat of such deposition. 

 The endosperm, however, a tissue external to the seedling may be the seat of 

 deposit of such reserves. It is unnecessary for us to enter on the discussion of 

 such purely morphological matters as the difference between endosperm 

 and perisperm, nor need we concern ourselves with the question as to why 

 some plants deposit their reserves wholly or in greater part in endosperm, or 

 only in cotyledons or in both situations. For the purposes of physiology it is 

 sufficient for us to know that such reserves are placed in the neighbourhood of 

 the growing parts of the seedling. It is important, however, that we should 

 become acquainted with the chemical nature of these reserves. 



In seeds we jneet with organic substances as well as minerals as reserves, 

 and the former we recognize as of two kinds, one nitrogenous and the other 

 non-nitrogenous. These three kinds of materials are not, however, always 

 stored up in seeds in the proportions in which the young plant makes use of 

 them. If that were the case the assimilated reserves at any given time 



