277 



The ethereal oils and resins are recognised as products of 

 excretion. But the significance of the relation of these bodies to 

 the economy of the parent is not clear. 



The significance of the presence of caoutchouc in plants is, 

 perhaps, still less clearly understood. Caoutchouc occurs in the 

 latex of plants of different natural orders, among which are the 

 Eiiphorbiaceac, including Manihot, Ceara, and Hevea, Para rubber, 

 the Urticaceac, containing Ficus, Rambong, and Castilloa, and 

 the Apocynaceae of which Willnghbeia and Leucouotis, Borneo 

 rubbers, are members, along with the various natural orders 

 which contain numerous species yielding so-called " gutta-percha." 

 Latex is the name given to a fluid which is either watery or 

 viscous, colourless, white, yellow, orange or red, and is contained 

 in specialised cells, called latex tubes. The cow tree of Venezuela 

 {Galactodendron utile) yields a sweet milk of good flavour; the 

 dried tatex of the Poppy (Papaver somniferum) is the opium of 

 commerce; the milky Agarics, fungi of the mushroom type, yield 

 white, orange or red latex; and may other species occur which are of 

 irrterest or of economic importance. 



Latex is an emulsion of various substances in a water-basis ; 

 these are resins, caoutchouc of different kinds, oils, tannins, proteids, 

 sugars, starch, alkaloids, ferments and salts. The tubes in which the 

 latex occurs are divided into two classes according to their mode of 

 origin, viz. — laticiferous vessels and laticiferous cells; the former 

 arise by the fusion of independent cells, this class including Mauihot 

 and Hevca, while the latter originate by the growth of special cells 

 which are said by some to be differentiated in the undeveloped 

 embryo of the seed. These tubes, when fully formed, are living 

 cells connected by branches and frequently forming a close network; 

 they occur in all parts of the plant. 



The tubes, when present, are associated in the stems and leaves of 

 plants with those special tissues to which the function of conducting 

 plastic food-material is ascribed. And this close association, coupled 

 with the richness of the latex in food substances, such as proteid, 

 starch and suear, suggests at once that the latex tubes function as a 

 conducting system by means of which food material is conveyed 

 from one part of the plant to another. There is other evidence in 

 support of this suggestion. For example, where latex tubes occur, 

 those particular tissues which are normally concerned with the con- 

 duction of so-called elaborated food-material are deficient and are 

 frequently badly developed. Again, in Euphorbia, as the young plant 

 commences to develop in the S3ed the latex becomes poorer ; when 

 it has germinated the latex grows richer. And abnormal conditions 

 which stop certain of the life-processes, notably that of assiniilatioji, 

 make the latex poor. 



Assuminr, then, fiat tht: tubjs serv<3 to co.iduct t\;od-material 

 in the plant, the question arises " Is the late c actually in circulation 



