REPRODUCTION OF PLANTS. 97 



force propelling it outwards ; and the reverse 

 with regard to the stem. Although it is plain 

 that gravity is the efficient cause in establishing 

 the directions of the stems and roots of plants, 

 it is not so easy to understand the manner in 

 which it produces opposite effects on these two 

 organs. Various theories have been formed to 

 account for this, and the most plausible is that 

 which ascribes it to the different manners in 

 which the newly developed tissues are added to 

 the root and stem. In the root the addition is 

 almost entirely confined to the very extremity, 

 while the stem continues to increase for some 

 time through its whole length. Hence it is 

 supposed that the soft materials continually 

 deposited at the extremity of the root must ever 

 be tending downwards from the effect of gravity 

 alone. (Henslow's Prin. of Botany, p. 292.) 



Is it not probable that we may find the agency 

 of light connected with the fact of the plumule 

 ascending ? 



72. The reproduction of the tribes of the 

 Cryptogamia takes place in a very different 

 manner from that of the flowering plants. In 

 all of them it occurs spontaneously, and without 

 any contact between one part of the plant and 

 another. At the season of the year when the 

 H 



