34 BIOLOGY FOR BEGINNERS 



the seed must get from the soil before it can grow. The growth of 

 a seed is a very wonderful process. Though inactive, dry, and 

 apparently dead the protoplasm is really alive and only awaits 

 favorable conditions for growth to begin. 



The insoluble, stored foods must be digested by the embryo, 

 made soluble, united with the water which has been absorbed from 

 the soil, and assimilated, to form all the new kinds of tissue in 

 the growing seedling. It may seem strange to speak of a seed as 

 digesting food, but there is a substance (diastase) in the seed, 

 which digests its food just as truly as the fluids of our stomach 

 digest ours. Here, then, are digestion, absorption, and assimila- 

 tion going on in the seed as it begins to grow. If the food stuffs 

 in the seed were not stored in a dry and insoluble form, they 

 would dissolve and decay. It is necessary, therefore, if a seed 

 is to keep over winter, that its food must be both dry and 

 insoluble. 



EXAMPLES OF SEED STRUCTURE 



Each seed differs somewhat from the general description just 

 given; the parts of the embryo may be well or poorly developed; 

 the number of cotyledons may vary; and the endosperm may be 

 lacking altogether. 



All that is necessary for a true seed is the embryo, stored food, 

 and protective coverings. These are often very different in 

 structure, to adapt them to various surroundings. 



The bean is presented as an example of a dicotyledonous seed 

 without endosperm, while the corn is taken as a type of a mono- 

 cotyledonous seed in which there is a very large endosperm. 



The Bean. .External Structure. This familiar seed is usually 

 kidney-shaped or oval in outline, several being borne in a pod, 

 which is the true fruit of the plant. 



The testa is usually smooth and may be variously colored; on 

 the concave side it bears a scar (hilumj, marking where it was 

 attached to the pod. By means of this attachment it also received 

 nourishment when growing on the parent plant. 



