CIRCULATION. 113 
admirable structure of which we have before mentioned, that this sap 
takes its way. These fibres are very rich in a mucilaginous and 
albuminous matter. Some physiologists consider that there are 
additional vessels as principal and essential reservoirs for the 
elaborated sap, but it must be confessed that the subject is very 
obscure. The circulation of sap is in its most active state in 
spring time, when the plant is gorged with nutritive matter pre- 
served in deposit during the winter. It is then full of liquid, and 
in some plants the juices flow from the slightest incision. In spring, 
according to the poetical expression consecrated by use, the vine 
and other plants bleed; but when the leaves are fully developed, 
the active evaporation which takes place on their surface impels 
the liquid to the extremities of the vegetable, whence it exhales 
in vapour; they will no longer bleed when wounded. 
When the branches develop themselves and consolidate, the 
movement of the sap becomes slower; it is sometimes roused 
towards the end of summer, when, the spring having been pre- 
mature, the materials which the plant has elaborated for the 
vegetation of the following year have been set to work before their 
time. After the fall of the leaf, and when the approach of winter 
lowers the temperature, the movement of the sap is stopped 
entirely ; the tree arrives by little and little at a state of almost 
absolute repose: this is not death, but life, which awaits its re- 
awakening. 
It thus appears that all plants in a healthy state must be so 
situated, as to be able to absorb from the soil surrounding them 
the elementary bodies which constitute sap, namely, carbon, oxygen, 
hydrogen, and nitrogen, which are capable of forming a number of 
secondary combinations; as well as those which are necessary to 
the growth of certain families. The principal form in which these — 
bodies are absorbed is, as water, carbonic acid, and ammonia. 
Water is supplied to plants in the form of rain and vapour ; 
according to Schleiden, by far the greater part being due to the 
latter process, 
Carbonic acid, which supplies plants with the carbon of their 
tissues, is supplied by the respiration of man and animals: by com- 
bustion, and by- the decomposition of saccharine matter, volcanic 
action, and hot springs. 
I 
