478 MECHANISMS FOR CONVEYANCE TO AND FRO. 



in fig. 128^. The young branches rooted to the wall are elliptical in transverse 

 section, being always somewhat compressed on two sides. The outer portion 

 is composed of the epidermis, two layers of elastic parenchymatous cells below 

 it, and a layer of green cells. Then comes the ring of soft bast, outside which 

 bundles of hard bast are deposited; then the rings of cambium and wood, and 

 in the centre a large pith, which sends out single- and double-rowed medullary 

 rays through the wood ring. So far the arrangement of the various tissues 

 exhibits nothing particularly noticeable, and coincides with that in the young 

 branches of numerous woody plants. But tracts of cambium cells are subse- 

 quently formed in a remarkable manner on the inner side of the ring of wood 

 adjoining the pith; these develop wood towards the exterior and soft bast on the 

 interior. The constituents of the soft bast — sieve-tubes and bast parenchyma — 

 form quite conspicuous bundles which project into the pith, and being here 

 excellently protected against lateral pressure, can perform their duties undis- 

 turbed. Should the conducting cells and sieve-tubes of the outer ring of bast 

 not perform their duty, these inner ones still remain for the transmission of the 

 plastic materials. 



Thus the various arrangements of the constituents of the stem, and especially 

 the position of the channels for the streams of materials formed in the green 

 tissues, is in part accounted for by the protection gained against the injurious 

 action of external pressures and strains, and these act in the most varied way 

 on the exterior, according to the individual mode of life of the plant and the 

 conditions of its habitat. 



It is to the growing parts of plants, the extremities of roots and branches 

 especially, that organic matter is conveyed; also to places where the cells already 

 present become stimulated to fresh activity, where dead and dying cells are 

 replaced by fresh ones, and where constructive materials in sufficient quantity 

 must be at hand. Then again, the travelling substances are directed to those 

 places where protective and attractive agents are necessary to contribute indirectly 

 to the maintenance and multiplication of the species, and where a consumption 

 of materials is connected with this protection or allurement. It is thus of 

 importance, for example, that the honey excreted from certain parts of flowers, 

 which serves as food to the insect guests which effect fertilization, should be 

 always present in sufficient quantity, and that in case of its removal from the 

 receptacles by bees or butterflies, it should be immediately replaced by fresh 

 supplies. Care must also be taken that pepsin and other substances necessary 

 for digesting prey should be abundantly present in the pitfalls and other 

 mechanisms which serve for the capture of animals, and that a sufficient quantity 

 of alkaloids and bitter substances, which prevent ruminants from devouring 

 foliage, should be brought to the right places at the right time. In connection 

 with the process of rejuvenescence and multiplication also, it is necessary that 

 those cells and groups of cells, which become detached from the plant-shoot and 

 journey out into the wide world as spores and seeds, should be equipped with 



