FUNCTIONS OF PROSENCHYMATOUS CELLS AND VESSELS. 755 
ducing in their contents the various organic compounds which 
are concerned in the development of new tissues, and in the 
formation of others which have been termed secretions. (See 
Respiration and Assimilation.) In old cells the secretions of the 
plant are also, in part, deposited. 
2. Functions oF PRosENCHYMATOUS CELLS.—Prosenchy- 
matous cells are especially adapted by their construction and 
mode of combination into a tissue, for giving strength and 
support to plants ; and there can be no doubt but that this is 
one of the offices which they perform. In a young state, also, 
before their walls are thickened, they appear to be the main 
agents by which the fluids absorbed by the roots are carried 
upwards to the leaves and other external organs, to be elabo- 
rated by the agency of heat, light, and air. The experiments 
of Hoffmann, Unger, Knight, Dutrochet, and others, seem to 
prove this. Thus, Hoffmann, by placing plants in such a posi- 
tion as to cause them to absorb a solution of ferrocyanide of 
potassium, and then adding a persalt of iron to sections of them, 
found that the prussian blue which was formed by the reaction 
of the chemical agents thus applied was principally deposited 
in the prosenchymatous cells. Unger also came to the same 
conclusion, by causing plants to absorb a coloured vegetable juice, 
and tracing its passage. Knight and Dutrochet cut a ring of 
tissue out of the stem down to the duramen, with the result 
that the leaves withered, and the tree subsequently died. But 
other experimenters, such as Link, Rominger, and Herbert 
Spencer, have arrived at opposite conclusions. (See Pwnctions 
of Vessels.) 
3. Functions oF VessEets.—The functions of the spiral, 
annular, reticulated, pitted, and scalariform vessels have been a 
subject of much dispute from an early period, and have been 
repeatedly investigated. Hales, Bischoff, and others came to 
the conclusion that these vessels were carriers of air, and it is 
certain that air alone is found in old vessels ; while Dutrochet, 
Link, Rominger, &c., believed that their essential function 
was to carry fluids from the root upwards, which views from 
recent observations appear to be correct. According to Link, 
when plants are watered for several days with a solution of 
ferrocyanide of potassium, and afterwards with a solution of 
persulphate of iron, prussian blue is found in the vessels, and 
not in the prosenchymatous cells, as the experiments of Hoff- 
mann, alluded to in speaking of the functions of prosenchyma- 
tous cells, seem to indicate ; and, more recently, the experiments 
of Herbert Spencer, conducted with great care, tend to show 
that in young plants at all events the vessels are the chief sap- 
carriers whence the fluid exudes into the surrounding prosen- 
chyma. From this it is clear that the constituents of the xylem 
are sap-carriers. 
Functions of Laticiferous Vessels.—The physiological import- 
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