ANATOMY OF TISSUES. 73 
or nucleus, but are filled with a milk-like fluid, generally white, 
seldom yellow or reddish yellow, which exudes freely when 
the plant is cut or broken. This liquid contains various sub- 
stances, some of which are dissolved, others in the form of small 
grains or drops. These substances are mineral salts, sugar, 
gums, starch grains, albuminous matter, alkaloids, acids, wax, 
fats, etc. Many narcotics used in medicine are obtained from 
the contents of these tubes. Their principal commercial pro- 
ducts are opium, from Papaver somniferum, and caoutchouc, 
from various plants. These tubes are richly branched and ex- 
tend through a large portion of the plant, in some instances 
forming a continuous system, which is frequently referred to as 
the laticiferous system. The walls consist of pure cellulose, 
and are thin, or only slightly thickened, so that when they are 
FIG. 35. 
Tannin tubes in the rind of the oak. —(Th. Hartig.) 
cut or broken the tension of the surrounding tissue readily 
presses out their contents, in the form of drops. Accord- 
ing to their form and manner of development these tubes 
are divided into two classes, articulated and non-articulated, 
or multicellular and unicellular. The articulated arise from a 
series of elongated cells which coalesce by perforation of their 
