FRUITS AND SEEDS 245 



supply of oxygen. If, therefore, we wish to keep seeds healthy 

 during their period of rest, we must avoid dampness. The 

 latter will be the less harmful the lower the temperature is 

 and the less oxygen there is present. If seeds are not properly 

 attended to, they become musty and ultimately mouldy. If 

 we have the choice between a warm place liable to consider- 

 able variations of temperature and a uniformly cool place, 

 we ought always to choose the latter, regardless of the fact 

 that the seeds might be exposed to a frost. Most seeds of 

 our cultivated plants will resist comparatively low tempera- 

 tures if they are kept completely dry The storehouses for 

 seeds should be dry, eool, and airy. 



During the process of germination three definite periods 

 may be recognised. First there is the swelling of the seed. 

 This process may be looked upon as a more or less mechanical 

 one, and is accompanied by a rise of temperature (due perhaps 

 to the condensation of moisture). This absorption of water 

 brings about the second phase, the solution of the food material, 

 which is a chemical process caused by the action of certain 

 ferments, and causing the third change to take place, namely, 

 the increase and expansion of the embryo. 



The rapidity of the germination depends mainly upon the 

 rapidity with which the water is taken up by the testa. lu 

 some plants the epidermal cells of the testa, and sometimes 

 also the adjoining cells, have a palisade-like arrangement, and 

 take up water so rapidly that in a few hours they form a 

 mucilaginous coating to the seed. This is the case with the 

 seeds of pears and quinces, and with linseed. In other cases, 

 the structure of the testa is so firm, and the cuticle covering 

 its epidermal cells renders it so impervious to water, that 

 perfectly sound seeds may lie for years in a damp soil without 

 germinating. This is often the case in leguminous plants 

 (Eobinia, Clover, &c.). In the case of the latter it has been 

 shown that the palisade-like epidermal cells of the testa, which 

 contain the colouring matter of the seed, are so impervious to 

 water, that, after lying in water for several years, the seeds 

 were still alive and had not germinated. 



Sometimes the loss which occurs owing to the non-germina- 

 tion of a considerable percentage of the seeds may be avoided 



