MANUAL OF AGPJCULTUEE. 239 



water, and absorbing nutrition from that medium, e.g., duck 

 weed; aerial, as in the family of orchids, which in the tropics are 

 attached to foliage, and their roots hang in the air, whose mois- 

 ture they absorb, and against the branches, whence they derive 

 food from the tree's decay ; and 'parasitic, those fastened to the 

 substance of other plants whose sap they absorb, and they have 

 no direct connection with the soil. Such are moulds, the dodder, 

 injurious to clover, and the festive mistletoe. 



The root functions accordingly are, with exceptions to fix the 

 plant, to absorb nourishment from the soil, and occasionally, as 

 in the turnip, to serve as a magazine of nourishment for the 

 plant's use in promoting its growth at a future season. The 

 absorbent cells of roots would appear to possess a power of selec- 

 tion and rejection over suitable and injurious food constituents. 

 The absorbtive process is considered to be that of endoniosis, 

 which signifies the property of gases and fluids, enabling them to 

 pass through certain membranes in order to mingle with other 

 fluids and gases possessing different densities and compositions. 

 " Exosmosis " expresses the converse process. 



The stem is the organ which bears the leaves and flowers; and, 

 like the root, it^assumes all possible phases of modification. Some 

 stems are long, others short, so as to be scarcely seen above 

 ground ; some burrow to a varied extent under ground ; whilst 

 others again, for support, have to cling to stronger neighbours ; 

 familiar specimens, whereof respectively are ordinary trees and 

 grain plants, the turnip, the quicken grass and potato, and ivy. 

 The tuber of the potato is in fact a stem, its eyes the buds, pro- 

 ducinc'" branches and leaves. Stems are divided into three g'reat 

 classes, viz., exogenous, endogenous, and acrogenous. A stem 

 of the first of these divisions increases in diameter by the addi- 

 tion of matter to its outer circumference. A cross section shows 

 in its centre the pith, with lines radiating from it to the circum- 

 ference or bark, which are called medullary rays, and concentric 

 rings round the pith, each marking a year's growth. Outermost 

 is the bark, easily separable from the wood proper. The annual 

 increase to wood and bark takes place immediately under the 

 latter in a layer of slimy substance termed the cambium. The 

 outer and newer wood is called the laburnum, the inner or heart- 

 wood, which is denser, generally of a darker colour, and through 

 which there is less sap circulation, is named the duramen. The 

 bark has three layers, — the innermost tough and fibrous, forming 

 in some i)lants, c.(j., llax, " the bast." 



An endogenous stem increases in diameter by the collection iu 

 its cellular centre of bundles of fibres and vessels, which swell 

 out and extend the outer circumference. In a-cross section we see 

 no pith, no concentric rings, no true separable bark, but on the 

 contrary, a hardened cellular mass of bundles of vessels and 



