11 



free. One of the most important of these immediate products is 

 starch, but there are many others such as oils, acids, &c. The leaves 

 therefore, perform the double functions of transpiration and assimila- 

 tion in addition to these, a true respiration similar to that of animals 

 takes place in them, the plant absorbing oxygen and exhaling carbonic 

 acid, and effecting transformations in some of the substances assimi- 

 lated during the day. This respiration is noticeable at night, but 

 is not so considerable as the absorption of carbonic acid during the 

 day. 



Having undergone these different processes in the leaf, the sap 

 descends through the vessels of the bark, and is distributed to the 

 different parts of the plant as required, a part of it finding its way to 

 the roots, where it provides for the elongation of these organs; and at 

 the same time a small quantity which escapes, helps to dissolve 

 certain mineral substances of the soil which are not soluble in water, 

 such as carbonate of lime and phosphates, and thus allows them to 

 enter the vine. 



It is evident from all this, that carbon, which is the most abundant 

 solid element in all plants, and which remains in the form of charcoal 

 when almost any vegetable substance is strongly heated without a 

 sufficient supply of air to burn it, is derived solely from the atmosphere, 

 and assimilated by the leaves, none of it being absorbed through the 

 roots. The only substances absorbed by the roots are water, ash, and 

 compounds containing nitrogen (ammonia and nitrates). These do not 

 form a large percentage, and if we except water, which is of course 

 necessary to dissolve the other substances, we shall find that not 8 per 

 cent, of the total weight of the plant comes from the soil, the remaining 

 92 per cent, being absorbed from the air. Strange as this may seem 

 it is not less true, no scientific fact being more clearly proved. 

 The different organs perform their functions in this manner through- 

 out the summer, the roots requiring moisture without the help of 

 which they would be unable to absorb the necessary mineral sub- 

 stances of the soil, or to replace the water lost by the plant through 

 transpiration. The leaves on their side require heat, light, and air. 

 Although it is necessary that moisture should be present in the soil, 

 it must not be so in too great a proportion as the vine requires air at 

 its roots as well. If the soil be swampy or sour, the vigour of the 

 plant will be greatly impaired, and it will fall an easy prey to any 

 disease to which it may be subject. 



