338 CONDUCTING SYSTEM 



of substances in solution, such as mineral salts (e.g. calcium malate in 

 species of Euphorbia, magnesium salts in Fieus elastica and many other 

 plants), proteins, sugars, gums, tannins or alkaloids (such as the 

 morphine contained in opium). Proteolytic enzymes are also known to 

 occur in the latex of certain plants (e.g. Carica Papaya and Ficus 

 Carica). Many additional facts concerning the qualitative and quanti- 

 tative composition of latex are recorded in a recent dissertation on the 

 subject by Molisch. 



The general conclusion to be drawn from the preceding statements 

 is, that latex must be regarded as a nutritive fluid, because of the large 

 quantities of plastic substances present therein, but that it nevertheless 

 also contains substances which are really of an excretory nature, or 

 which, in other words, represent by-products of metabolism. 



From a morphological point of view, it is necessary to distinguish 

 between two different types of laticiferous organs, namely, articulated 

 latex-tubes or latex-vessels and non-articulated latex-tubes or latex-cells. 

 Latex-vessels are characteristic of the Chicoriaceae, Campanulaceae, 

 Papaveraceae, Lobeliaceae and Papayaceae, also of many Araceae and 

 Musaceae, and of the genera Manihot and Hcvea among the Euphor- 

 biaceae. They originate from rows of meristematic cells, in which the 

 transverse septa become absorbed at an early stage of development. In 

 a few instances (e.g. Musa and Chelidonium), the transverse walls 

 persist, but are perforated by one or more large pores. Similar perfora- 

 tions occur, in all cases, on the longitudinal walls, wherever two latex - 

 tubes come into contact with one another. Typically, latex-vessels 

 develop numerous branches, which anastomose freely, so that a dense 

 network of tubes results (Fig. 137 b). Both the primary branches, 

 and the anastomoses, may arise either by the fusion of rows of 

 meristematic cells, or by the formation of lateral outgrowths which push 

 their way between the surrounding tissues. 



Latex-cells occur in the majority of Euphorbiaceae, and in the 

 Urticaceae (including the Artocarpaceae and Moraceae), Apocyna- 

 ceae and Asclepiadaceae. In these cases each tube arises from a single 

 mother-cell, which becomes enormously elongated and abundantly rami- 

 fied. Anastomoses are altogether absent, or at any rate infrequent (Fig. 

 137 a). According to Schwendener, the mother-cells of the latex- 

 tubes are clearly recognisable even in the very young embryos of the 

 Euphorbiaceae. Schwendener's observations have been confirmed by 

 Chauveaud, and extended with essentially similar results to the rest of 

 the above-mentioned families. The initial cells in question appear at the 

 inner margin of the primary cortical tissue in the first or cotyledonary 

 node, and by means of active apical growth soon develop into elongated 

 branching tubes. Their branches behave like the hyphae of a parasitic 



