CONCLUSION OF VEGETABLE TISSUES. 
113 
a tendency to unroll, but other cells, which are called 
porous ducts, are of great size, have their walls covered 
with large pores, and are not capable of being un- 
rolled. These are the tubes visible to the naked eye in 
almost every wood. There are other ducts found in all 
plants yielding a milky juice, called laticiferous or milk 
vessels by Schultz, who, from supposing them to 
exhibit a circulation of their contents, likened them to 
the capillaries of animals ; but by recent examinations, 
both the existence of the circulation, and the entire con- 
tinuity of the vessels, have been proved not to exist. 
Such, then, is a brief outline of the subjects that 
have occupied our attention during the first part of 
the course. One great object which I have kept in 
view throughout, has been that of endeavouring to 
impress on you the fact, that each cell of a plant should 
be considered as having an independent or individual 
existence ; that in one situation it may secrete colouring 
matter, in another starch, gum, sugar, oil, &c. ; and 
in another the material for the reproduction of its 
species. 
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