27 / 
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 
Euphorbiaceac, including Manihot, Ceara, and Hevea , Para rubber, 
the Urticaceac, containing Ficus, Rambong, and Castilloa, and 
the Apocynaceae of which Willughbeia and Lenconotts, 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 watt ry or 
viscous, colourless, white, yellow, orange or red, and is contained 
in specialised cells, called latex tubes. The cow tree of Venezuela 
( Galactodeudron utile) yields a sweet milk of good flavour; the 
dried latex 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 
interest 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 Manihot • 
and Hevea , 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 dose 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 r 
starch and sugar, suggests at once that the latex tubes function as a 
conducting system by means of which fcoi 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 s eed 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 assimilation 
make the latex poor. 
Assuming, then, that the tubes serve to conduct food-material 
in the plant, the question arises “ Is the late c actually in circulation 
