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Se ay poor bid Physiological Delany 
of the blood in animals and that of the sap in plants must therefor be s ieee 
- entirely abandoned. 
If now we inquire what are the causes of the movements of the © 
assimilated nutrient sap, it is impossible to give a definite answer, 
because the action of organised and living protoplasm introduces inde- 
-. terminable factors. It may, however, be assumed that the law of 
diffusion in its widest sense is one cause of these currents. The move- 
ment. in cells enclosed on all sides is brought about by two causes: the 
force of osmose and the ¢enston of the tissues, that is the force exercised 
on the conducting cells by the surrounding ones. The movement of the 
sap can take place more readily in intercommunicating tubes than in 
closed cells, as is seen in laticiferous vessels and sieve-tubes. Thus, for 
example, in the celandine, Chelidonium majus, the slight rise in tem- 
perature caused by the approach of the hand at the distance of an inch 
is sufficient to cause a current of latex towards the colder parts which 
may be perceived under the microscope. This movement is obviously 
occasioned by the pressure or tension which the swelling of the neigh- 
bouring cells exercises on the thin-walled laticiferous vessels. Other 
forces besides those already mentioned operate in these tubes, especially 
the curvatures and torsions of particular parts of the plant ora enee | 
by the influence of light, gravitation, and the wind. 
The products of assimilation, when transferred to other 
cells, and when brought into actual use, undergo a variety of 
changes which are included under the term metastases. ‘Thus, 
for example, when the starch formed in the leaves of the 
potato is to be transferred into the tubers as a_reserve- 
material, it must first be transformed into a soluble substance, 
vez. glucose. This glucose is distributed through the con- 
ducting tissues ; and being again transformed into starch in the 
cells of the tuber, all the starch gradually collects in these 
organs. By the close of the period of growth, all the chloro- 
phyll-grains are also dissolved and carried into the permanent 
organs, together with all the other substances which are sub- 
sequently to be employed. Many products of metastasis 
are of no further use in the construction of new cells, but 
remain stored up and inactive at the spots where they are 
produced. ‘These are termed secondary products of meta- 
stasts, and comprise the volatile oils, resin, caoutchouc, 
