142 CIECULATION OF THE SAP. 



leaves. The former, including Orobanche or broom-rape, Lathrsea or 

 toothwort, Ouscuta or dodder, derive nourishment entirely from the 

 plant to which they are united ; while the latter, as Loranthus, Viscum 

 or mistleto, Myzodendron, Thesium, Euphrasia, Melampynim, and 

 Buchnera, elaborate sap in their leaves under the action of air and 

 light. By this power of elaboration, the mistleto is able to grow on 

 different species of plants, as on the apple, beech, oak, etc. Some 

 parasites are attached by suckers to the roots of plants, as in the case 

 of Broom-rape, Toothwort, and Thesium, and are called root-parasites ; 

 while others, as Dodder, Mistleto, etc., derive nourishment from stems, 

 and are called stem-parasites. The specific names of many parasites 

 are taken from the plants on which they grow. The species of 

 Cuscuta or dodder inhabit all the temperate and warm parts of the 

 globe, and are peculiarly destructive to clover and iiax. They are 

 produced from seed which at first germinates in the soil like other 

 plants ; but after the stem has coiled closely round another plant, and 

 become attached to it by means of suckers, then all connection with 

 the soil is severed, and the Dodder lives as a true parasite. A re- 

 markable genus of parasites, called RafElesia, has been found in Sumatra 

 and Java. The species are leafless, and produce brown-coloured flowers, 

 which are sometimes three feet in diameter. On account of their only 

 producing a flower and root they are denominated Rhimnths (g/'^a, a 

 root, and a,v6og, a flower). 



2. — Absorption and Circulation of Fluids. 



While the leaves and other aerial organs of plants have the power 

 of absorbing fluids, it is chiefly by the roots that this process takes 

 place. The cells of the spongioles or fibrils of the roots are covered 

 by a very delicate membrane (p. 38), which allows the imbibition of 

 fluids to proceed rapidly ; and as additions are made to their extremi- 

 ties, they are constantly placed in circumstances favourable for the 

 reception of fresh nutriment for the plant. Animals having the 

 power of locomotion are enabled, as they exhaust the nutritive matter 

 of one locality, to remove to another. Plants having no provision for 

 locomotion would perish, after taking up all the nourishment in the 

 soil in their immediate neighbourhood, were it not that the roots spread 

 over large areas in search of food. The nutritive materials in the soil, 

 partly derived from the decomposition of its organic and inorganic 

 materials, and partly from the atmosphere, are supplied to the roots 

 in a state of solution ; and as the substances in the cells of plants are 

 usually colloid and denser than the external liquid crystalloid matters, 

 a process of endosmose takes place by which the latter pass in large 

 quantities into the cell through its membranous covering, while a 

 small portion of the former is excreted by exosmose. These move- 



