10 



sap, and found that it was equal to a pressure of about 10 Ibs. per 

 square inch. When the plant is in full activity, the rise of sap is 

 still greater, as there are more factors to promote it. 



The leaves exert a very considerable influence. The leaves of all 

 plants transpire or exhale through their stomata or breathing pores 

 large quantities of water. Those of the vine are no exception to this 

 rule. Hales found that a vine with a leaf surface of 1 ,820 square inches, 

 or 120 medium-sized leaves, exhaled from 5 oz. to 6 oz. of water daily 

 under ordinary circumstances. This amount, however, is not con- 

 stant it varies with the amount of light the plant is exposed to and 

 the moisture of the air. Plants do not transpire in the dark nor in a 

 very faint light. This function is more acted upon by light than 

 heat. It makes room for the fresh sap which is being continually sent 

 up by the roots. This sap becomes concentrated by the loss of 

 water, and by osmosis favours the rise of a fresh quantity. Osmosis 

 is the name given to a physical phenomenon which takes place when 

 two liquids of different density are separated by a thin membrane ; 

 both liquids traverse the membrane, but the less dense does so to a 

 far greater extent than the denser one. This process of osmosis 

 takes place throughout the whole of the plant through the cell 

 membranes from the root where the liquids enter the absorbent hairs, 

 in the form of a thin watery solution, to the highest parts of the 

 plant. It is the principal cause of the rise of the sap, which is also 

 promoted by capillary attraction in the long vessels of the plant. 



It is unnecessary to enlarge upon the cause of this rise of the sap ; 

 it is sufficient for us to know that it does rise, and not only does it 

 rise but it circulates. 



After entering the roots through the microscopic absorbent hairs 

 in the form of a thin watery solution, it rises through the vessels of 

 the young wood and finally reaches the leaves, to all the tissues of 

 which it is distributed by the veins. In the leaves it is not only 

 concentrated by loss of water, as we have already seen, but it 

 becomes enriched with many new substances derived from its contact 

 with the atmosphere. 



Under the influence of the light of day the chlorophyl or green 

 colouring matter is capable of decomposing the carbonic acid of 

 the air and taking the carbon with which it forms immediate 

 products, which will in turn undergo transformations in order 

 to form the different tissues, &c., of the plants, the oxygen being set 



