128 A TEXTBOOK OF GENERAL BOTANY 
through the resin ducts of pine trees, after which the turpentine 
exudes and is collected. Various kinds of varnish and other 
resins are obtained by the same method from other trees. 
Resin ducts are, essentially, long passages surrounded by glan- 
dular cells. They occur not only in stems but also in other parts 
of plants. 
Internal glands. Resin canals are one type of internal gland, 
of which a number of other forms occur in various plant organs. 
Internal glands frequently contain essential oils. ‘hese oils are 
volatile and are usually very odoriferous. Well-known examples 
are eucalyptus oil and the oil from orange peel. The latter 
occurs in large oval glands. These glands originate in the split- 
ting apart of certain cells, but are formed largely by the breaking 
down of cells containing the oil. The disintegration of the cells 
brings the oil into the large cavity of the gland (Fig. 122). 
Latex tubes. Milky juice (latex) is found in long, branching 
tubes known as latex tubes (Fig. 123). In many cases neighbor- 
ing tubes become connected, thus forming a network (Fig. 124). 
When these tubes are cut, the latex exudes as a white, watery 
juice with a milky appearance. Rubber, gutta-percha, opium, 
gum chicle (the chief base of chewing gum), and other valu- 
able substances are derived from coagulated latex. The tapping 
of rubber trees consists in cutting the tubes and allowing the 
latex to exude. 
Latex tubes are formed in two very different ways. In some 
cases rows of cells lose their cross walls and become trans- 
formed into latex tubes, as in Pard rubber (Hevea brasiliensis). 
In other cases the tubes are not part of the tissues produced 
from either the apex of the stem or the cambium, but result 
from the growth of tubes found in the seedling. As the stem 
grows in length, or as other organs are formed, these latex 
tubes also grow and push their way between the cells of the 
newly formed tissues. 

