CHAPTER XIII 

 GENERAL STRUCTURE OF ROOT AND STEM 



The Development of Different Tissues. — The de\'e]opment of the stem 

 commences with the formation of the embryo, by the process explained 

 at the beginning of our study of the seed. So long as the cells produced 

 by this process are the same in kind, the body consists of but one tissue; 

 but through differentiation and specialization among them, difi'erent 

 tissues are soon developed. 



Meristem. — The power of cell-di\'ision and growth is lost by most 

 tissue after a time, while in other parts it persists permanently. Any 

 tissue or portion of tissue which possesses such power is called Meristem. 

 Tissue may cease finally to exert meristematic power, or it may resume 

 such power after a time. All meristematic processes cease upon maturity 

 of seed, recommencing with germination. 



Degree of Development Attained by the Embryo in the Seed-condition. — 

 The point reached in the development of any plant-body in the embry- 

 onic condition — that is, at the maturity of the seed — does not depend 

 in any degree upon the amount or kind of tissue or tissues developed, 

 but altogether upon the habit of the particular class of plant. In some 

 embryos, tissue differentiation cannot be seen to have taken place at 

 the time of separation from the parent, while in others it has progressed 

 very far, though never (unless germination has occurred) to the pro- 

 duction of a true root. It is impossible, therefore, to fix upon any 

 particular developmental stage of stem-structure as distinguishing the 

 ungerminated embryo from the germinated plantlet. In the following 

 sketch of its development, then, no note is taken of the resting period 

 in the seed-stage, but the process is followed as though it were con- 

 tinuous from fertilization through germination and into the mature 

 condition of the plant. 



Although of primary importance scientifically, and of great interest, 

 the phenomena of germination are not important from the standpoint 

 of pharmacognosy, and a mere outline of them is here given. 



Vitality of Seeds. — Animation is probably not entirely suspended 

 during the resting period of the seed. That is, there is an apparent 



