ARRANGEMENT OF THE LATICIFEROUS TUBES. 439 



there at an acute angle. At the next point of insertion of a leaf, which they reach on 

 their way, many of these branches, preserving the same position relatively to the surface, 

 bend out into the petiole, pass through this to the lamina, and here divaricate near the 

 epidermis of the lower surface. The great majority of the branches, which are dis- 

 tributed and terminate in this region, certainly belong to the hypodermal system in 

 question. The latter stands in no other connection with the main trunks belonging to 

 the bundles, than that which depends on its points of origin in the nodes ; it is separated 

 from them in the internodes by many layers of lacunar cortical parenchyma. Its presence 

 may always be recognised, even on the most superBcial observation, by the fact that from 

 almost every slight prick into an internode, though far from penetrating the cortical 

 parenchyma, drops of latex exude. The great majority, at any rate, of the Tithymalus- 

 EuphorbisE have essentially the same arrangement of the laticiferous tubes; e.g. E. 

 Cyparissias, sylvatica, Characias, Peplus, Lagascae, also E. Myrsinites, which is charac- 

 terised by especially large and numerous tubes; many differences occur, it is true, 

 according to the particular species, which chiefly affect the greater or less frequency 

 of hypodermal tubes, the presence or absence of medullary ones, &c., and may here be 

 passed over. 



No accurate investigations of the laticiferous tubes of other Euphorbiaceae exist. 

 Hanstein says : ' Where the development of the latex itself is inconsiderable, as in 

 Ricinus, Mercurialis, and other genera, there we find its vessels also less abundantly 

 distributed, and less conspicuous. They possess but scanty ramifications and anasto- 

 moses, but on the other hand they have more strongly thickened walls.' Vogl mentions 

 the laticiferous tubes in the outer and inner cortex of Hippomane Mancinella, and calls 

 attention to the great similarity of those of Hura crepitans with those of the succulent 

 Euphorbiae. 



8. The laticiferous tubes of the TJrticacesB, ApocynesB, and Asclepiadese, agree, so 

 far as investigations extend, with those of the Euphorbiae in all essential points, as regards 

 both their form, structure, and ramification, and their development and distribution. 

 This agreement becomes especially conspicuous if the succulent leafless Asclepiades, of 

 the genera Ceropegia and Stapelia, be compared with the Euphorbiae of similar habit. 

 On the average the laticiferous tubes of the families in question are narrower and 

 thinner-walled than those of the Euphorbiae, but very thick ones occur, e. g. in species 

 of Nerium and Ficus. With respect to the abundance in which they occur in the cortex 

 and pith of the stem, the divarication of their branches in the parenchyma of the leaf, 

 &c., the same differences between individual species of each family prevail, as within the 

 genus Euphorbia. While, e.g. in thick-leaved species of Ficus, they extend their 

 branches abundantly through the parenchyma of the leaf, up to the epidermis, in the 

 leaf of Humulus, according to Hanstein, they are confined to the vascular bundles, and 

 do not extend into their ultimate ramifications. Especially abundant subepidermal rami- 

 fications are described by Tricul in the leaves of the Asclepiadeae, Echites peltata, and 

 Arauja sericophora. 



Among Asclepiadeae and Apocyneae, a larger number of forms have been investigated, 

 especially by TrScul, e. g. Hoya carnosa, species of Asclepias (A. Cornuti, curassavica, 

 &c.), Physostemma, Centrostemma, Cryptostegia, Stapelia, Ceropegia, Echites, Arauja, 

 Nerium, Vinca, Apocynum, Plumiera, Tabernaemontana, and many others. 



Among the Urticaceae, more accurate investigation has been chiefly confined to species 

 of Ficus (F. carica, elastica, repens). Minute comparative investigations on the course 

 and development of the tubes are however still to be desired, not only for the orders 

 last mentioned, but also for the Asclepiadeae and Apocyneae (cf. p. 198). 



