470 MECHANISMS FOB, CONVEYANCE TO AND FRO. 



one-half of the vascular bundles, the so-called bast portion, while the other half, the 

 so-called woody portion, consists of woody cells intermingled with lignified tubes, 

 and other water-conducting elements (see figs. 125^' ^°' ^^). 



Laticiferous tubes form a fourth mechanism for conducting away the products 

 of the green cells (fig. 126^). These are thin- walled, much branched, frequently 

 anastomizing, tubular structures which seem to penetrate all the parts of the plant, 

 leaves, stem, and roots, without much regularity. 



They may be divided, according to their development, into laticiferous vessels 

 and laticiferous cells. The former are produced from rows of cells, whose partition- 

 walls have become obliterated, so that the rows of cells have become converted into 

 tubes; the latter arise from isolated cells, at first very small, but which elongate 

 enormously, become much branched, and whose branches penetrate between the 

 cells of other tissues just as the hyphse of parasitic fungi grow through the tissues 

 of their host-plants. Laticiferous tubes are not to be found in all plants. They 

 are particularly abundant in species of Spurge, some thousand species of Compositae, 

 for example, in the Salsify, Lettuce, and Dandelion ; in the Oleander, many 

 Asclepiadese, Papaveracese, and Artocarpeae. In the gigantic trunks of tropical Fig- 

 trees, the latex often wells up in large quantities from rifts in the bark which have 

 arisen spontaneously, and thickens into long strings and ropes of india-rubber 

 hanging down like a mantle. 



The Cow Tree of Venezuela (Galactodendron utile) is especially worth noticing 

 here; when pierced, a quantity of sweet, delicious milk pours out from it, also 

 Collophora utilis of the Amazon, from which is obtained a viscous latex, used as a 

 medium for colouring matters; finally the poppy {Papaver somniferum), whose 

 dried latex is known as opium. In the majority of cases the latex is white, but in 

 Papaveracese other colours are also to be found; thus the Celandine (Chelidonium 

 TTiajus) contains an orange, and the Bloodwort (Sanguinaria Canadensis) a blood- 

 red latex. The milky Agarics (Lactarius) contain partly white, partly sulphur- 

 yellow, partly orange, and vermilion latex. 



In the foliage-leaves the laticiferous tubes run with the vascular bundles, and 

 occasionally replace the bundle sheath; at least, the bundle sheath is defective, and 

 only very incompletely formed where the laticiferous tubes adjoin the vascular 

 bundle. It has also been observed that in the stems of the Asclepiadese, where 

 the laticiferous tubes are abundantly developed, the sieve-tubes are much reduced, 

 and it is therefore supposed that the various mechanisms for conducting away 

 materials are sometimes able to mutually replace one another. It must, moreover, 

 be expressly noted here, that the laticiferous tubes do not serve exclusively to 

 carry away the materials manufactured in the green cells; they are used, under 

 certain conditions, and at certain times, as receptacles for reserve materials, exactly 

 as the medullary rays, sieve-tubes, and bundle sheaths which in the winter, when 

 the decomposition of carbonic acid, and the formation of carbohydrates in the green 

 cells have ceased, and when generally there is nothing to remove, function as 

 reservoirs, in which stores are deposited until the following spring. The parenchy- 



