102 Circulation in V egetables. 
it regards CAPILLARY ATTRACTION, motion by winds, &c. 1 forbear 
to remark on them as causes of the descent of the sap. Of gravi- 
tation I would say, that we ought not assign it as an efficient cause,of 
action or motion, where vitality exists. 3. The growth of stems can 
be effected where no leaves are developed. ‘The vine of the potatoe 
growing in the dark or diffuse light of a cellar is almost destitute of 
leaves. It isa strong case to show that they are not required for as- 
similation. Jn all cases, the elongation of the branch and growth of the 
leaf take place simultaneously. 4. The evaporation of a large por- 
tion of the watery part of the sap so far inspissates it, that the descent 
becomes difficult. Gardeners know that watering delicate plants with 
barn yard water clogs the vessels and they die if the practice be long 
continued. Ifit be diluted with pure water, the practice is highly bene- 
ficial, showing us that the water is not poisonous. ‘The phenomena of 
vegetable nutrition and assimilation may be explained without bring- 
ing in the agency of the leaves. The spongioles takes up water, more 
or less loaded with foreign matter, as carbon, common air, ammonia 
and other alkaline salts. As soon as the fluids come in contact with the 
vegetable structure they are more or less changed. In their pro- 
gress upward, especially in herbaceous plants, they are still more 
changed and while passing through the. neck. When the sap has 
arrived at the extreme branches, its assimilation is complete and 
a part of it is employed in effecting the extension of the branch- 
es or expansion of the leaves; the remainder is exhaled. ‘That 
the fluids are changed, in their upward progress, before they have 
reached the leaves is shown by the fact, that the sap of the sugar 
maple is sweeter in the higher parts of the trunk than in the roots. 
Its specific gravity is also greater, and it is to be remembered that 
the maple is yet leafless. The elongation of the branches is finished 
in about three or four weeks, and the leafing is perfected in the same 
time. ‘These two processes engross all the fluids and bring into ac- 
tion all the energies of the plant. Afterwards the sap is differently em- 
ployed, viz. in forming the annual layer and in developing the buds. 
I have been led to consider the leaves as organs of exhalation, and not 
of assimilation. The power, namely, vegetable arritability, which 
causes the ascent of sap is.a vital property, and like animal irritabil- 
ity often acts periodically. ‘Thus after the long repose of winter, with 
how much energy do vegetables carry on their functions on the open- 
ing of spring. A few weeks only are required to transform a 
desert waste, into a beautiful garden. It is not improbable that the 
