STRUCTURAL BOTANY. 35 



sium, sodium, magnesium, etc., is not yet sufficiently ascer- 

 tained. The plant's nourishment is received either in the 

 gaseous or liquid form. In whatever mode imbibed, the 

 main vehicle of alimentation is water, which, as fluid or 

 as vapor, is in contact with the root, and as vapor continu- 

 ally surrounds the leaves.) 



The organs, which imbibe and convey these aliments, 

 are the Hoot, the Zea/ves, and the Axis {stem and branches). 

 But it is the root which takes in the greatest quantity. 

 From the root they are conveyed through stem and 

 branches, and to every point where new organs are to be 

 formed. The leaves absorb carbonic acid out of the air, 

 and eliminate oxygen and hydrogen, at least by day. 



I. THE EOOT. 



77, The EooT is an organ, which has its origin in the 

 radicle of the embryo. It generally grows downward, and 

 never produces leaves, but serves to fix the plant in the 

 earth, whence it derives its nourishment. It lengthens by 

 continued cell-multiplication, mainly at its lower extremity, 

 the parts, once formed, scarcely elongating aftei-ward. 

 Still the structure of the root agrees essentially with that 

 of the stem. 



We distinguish two principal sorts of roots: 1. Pri- 

 mary roots, which, start from the first joint of the stem of 

 the plantlet, springing from the seed ; and, 2. Secondary 

 roots, which occasionally proceed from other parts of the 

 stem. All stems, which creep on or under the surface of 

 the soil, are apt to Btrike root from almost every joint. 

 But even from erect stems we sometimes see roots spring- 

 ing — in Indian corn several inches above ground, and in 

 some plants high in the air, as in Pandanus. When 



