EXOGENS. 139 



ing organic matter (or humus) contained in the soil. 



Some plants grow without attachment to the soil, deriv- 



ing all their nutriment from 



the air, and are called Epi- 



phytes, (epi, upon; phyton, a 



plant,) from being generally 



found on trees. They differ 



from true parasites, since the 



latter prolong their tissues into 



other plants, and prey upon 



them. The Orchids may illus- 



trate the first, and the Dodder 



and Mistletoe the latter kind. 



The only structure capable of 

 effecting the chemical changes 



FIG. 58. Diagrammatic Section of a 



necessary tO plant nutrition is Flowering Plant, showing the different 

 . tissues. A. Ascending axis. B. De- 



the Chlorophyll, Which IS mOSt scending axis. s. Surface of soil. c. c. 

 . . . ,.. . -I Appendages, d. Growing point of stem. 



abundant in the leaves; hence ,. Epidermis . ././. stomata: g . Layer 

 the materials which supply food ? 



must be carried up to the ' m - pit ^ s P iral vessel > and dotted 



duct all air passages, r. r. Roots. 



leaves. The aSCent Of fluid t. Growing point of roots. w.Cap, (pil- 



eorhiza.) 



from the root to the leaves 



takes place by means of two distinct forces a pushing 

 force, caused by absorption by the extremities of the 

 rootlets, and endosmose (Chap. IV, Sec. 3) from cell to 

 cell; and a pulling force, produced by evaporation from 

 the surface of the leaves. 



The appendages of the root are the rootlets, and of 

 the stem, the leaves. Leaves are developed from the 

 nodes, and the internodes (Chap. VIII, Sec. 2) become 

 shorter toward the summit of the stem, which ends in 



