12 SEEDS AND SEEDLINGS 



enabling it to carry on an independent existence. We will 

 study the root, stem and leaf in the succeeding chaj^ters. 



The seeds of some plants retain their power to germinate 

 much longer than those of others. The harvesting of seeds be- 

 fore they are fully ripened is one of the most common causes 

 of loss of vitality, but there may be other factors which tend 

 to injure seeds. These facts make it very important that seeds 

 should be tested and their power of germination determined 

 before planting. (Chapter XVI.) 



Germination means the growth or enlargement of the young 

 embryo of the mature seed. We have learned that this occurs 

 under the proper conditions of moisture, temperature and air. 

 The young plant may be considered a seedling until it has used 

 the supply of stored food, and thus becomes an independent 

 plant, drawing its food from water, soil and air. 



Since we have three types of seeds it is very probable that 

 we will find three or more types of germination. If we examine 

 the seed of the bean again we will find a very minute opening- 

 near the hilum. It is the micro pijle (Fig. 2 a and h) through 

 which the water passes very readily. If the micropyle is closed 

 by wax the absorption of water will be much slower. The 

 embryo gradually enlarges and the first evidence of growth 

 that we see is the elongation of the radicle, which breaks through 

 the seed coat near the hilum and turns downward. The con- 

 tinued enlargement of the cotyledons and jDlumule forces the 

 seed coats off and the rapid elongation of that part of the stem 

 below the cotyledons carries them above the surface of the soil. 

 This elongated stem is not straight at first, but gradually appears 

 above the soil in advance of the cotyledons as a loop which 

 later becomes erect. (Figs. 7 and 8.) 



The seed of the castor plant (sometimes improperly called 

 castor bean) has a small mass of spongy substance (caruncle) 

 (Fig. 3 a and h) covering the hilum through which the water 



