22 PLANTS 



38. In many cases there are a large number of fibrous roots 

 which spring directly from the base of the stem instead of a 

 few larger roots. Such fibrous root systems occur principally 

 on low herbaceous plants which do not require an especially 

 strong supporting system. 



39. The structure of the root, in the main, resembles that of 

 the stem in that there is a central woody axis and an outer 

 bark; but there is no pithy core. The woody portion is not as 

 firm as that of the stem, but it is of ten very tough. The outer, 

 dead, protective layer of the bark of roots is also relatively 

 thin. 



40. In the preceding paragraphs some of the most evident 

 characteristics of ordinary plants have been noted for the 

 purpose of showing that the form and structure of plants is in 

 every case an adaptation to a certain end, and that even the 

 peculiarities of each kind of plant are special adaptations to 

 special ends. In continuing our studies we shall constantly 

 keep in mind this idea of adaptation to functions, that is to 

 say, at every point we shall seek to answer the question why? 



41. In the following paragraphs we shall take up for con- 

 sideration, in turn, the seed, the developing seedling and the 

 mature plant, studying in each case the structure and functions 

 of the principal sets of organs, so that at the end we may have 

 a fairly comprehensive idea of the life-history of a plant. 



Seeds 



42. Seeds present a remarkable diversity of form and struc- 

 ture, but there are usually two distinct sets of organs to be 

 recognized. The first of these are the seed coats, evidently 

 organs of protection, often consisting of an outer firm layer, 

 the testa, and an inner membraneous layer, the tegmen. How- 

 ever, the seed coats may be variously modified and cannot 

 be generally characterized. 



