102 Circulation in Vegetables. 





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 is a strong case to show that they are not required for as- 

 similation. In 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. If it 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 irritability, 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 



