138 GENERAL STRUCTURE OF ROOT AND STEM 



to essentially different climatic conditions; (2) a specific saturation, also 

 varying with different seeds — that is, the absorption of an amount of 

 water bearing a fixed ratio to the weight of the seed : (3) a partially fixed 

 degree of light exclusion; (4) the presence of free oxygen. 



The Process of Germination. — Under these conditions, ready prepared 

 nutriment is dissolved, other forms become digested by special vegetable 

 ferments (Enzymes) present, heat is developed, cell-propagation and 

 cell-growth take place, and the development and growth of a plant from 

 the embryo commence. By the growth of the embryo, the radicle is pro- 

 truded through the micropyle, the rest of the body soon following and 

 leaving the embryo free from its coats, or the body may remain enclosed 

 in the coats for some time. The» radicle, if it does not already point 

 directly downward, turns in that direction and develops into a root 

 (Figs. 417 and 419). The cotyledons may then separate completely, 

 leaving the plumule or second phytomer to develop from the apex, 

 between them (Fig. 416), or the cotyledons may remain in contact, and 

 the plumule or second phytomer burst forth from between the bases 

 of their petioles (Fig. 418). The end of the embryo opposite to the 

 radicle, if it does not already point upward, turns in that direction and 

 develops as the apex of the stem. 



The Epicotyl and Hypocotyl. — The stem above the cotyledons is called 

 the Epicotyl, that below them the Hypocotyl. 



Cellular Development and Growth. — The cellular nature of develop- 

 ment and growth demands a general knowledge of histology for their 

 understanding, so that we shall here consider, so far as possible, only 

 the gross results of the processes, or such characters of the root and 

 stem as can be demonstrated by other than histological methods. Such 

 references to cellular structure as are here necessary are given rather 

 figuratively than technically. The mode of growth in root and stem, 

 and the structures resulting, are sufficienti}' different to require separate 

 treatment. Although the forms of structure liere considered as applying 

 to the root concern only flowering plants and the very highest of the 

 cryptograms, yet the description is applicable to all roots used in 

 medicine. 



Structure of the Root. — Upon examining a transverse section of the 

 root in its rudimentary condition, it is possible to distinguish three 

 bodies of tissue exhibiting characteristic differences in their cellular 

 elements (P'ig. 420). 



The Plerom, Pleriblem, and Dermatogen. — The central portion is 

 occupied by a solid cylinder called the Plerom (a). Outside of this 



