CONSTITUENTS OF PLANTS. 135 



as 25 parts of dry potatoes. In the beet there are 18 or 19 part? 

 of sugar and 3 or 4 parts of cellular tissue ; the two per cent, 

 not accounted for consist partly of salts, and the remainder of 

 albumen. 



Turnips contain from 90 to 92 parts of water. From 23 to 25 

 parts of dry turnips contain 18 to 19 parts pectin, with very 

 little sugar, 3 or 4 parts cellular tissue, and 2 parts salts and 

 albumen. Sugar and starch do not contain nitrogen ; they exist 

 in the plant in a free state, and are never combined with salts, or 

 with alkaline bases. They are compounds formed from the car- 

 bon of the carbonic acid and the elements of water. In the 

 potatoe, these assume the form of starch, and in the turnip the 

 form of pectin. 



In the seeds of cereals we find vegetable fibrin, a constituent 

 containing sulphur and nitrogen ; in peas, beans, and lentils, we 

 find casein ; and in the seeds of oily plants, albumen and a 

 substance very analogous to casein. Casein and albumen have 

 the same composition as fibrin. 



Vegetable fibrin is accompanied by starch in the seeds of the 

 cereals ; the latter body occurs with casein in leguminous 

 plants ; but, in the oily seeds, its place is supplied by another 

 body devoid of nitrogen, such as oil, butter, or a constituent 

 resembling wax. 



It is obvious that we must furnish to plants the peculiar con- 

 ditions necessary for the development of these constituents, 

 according to our object in cultivation. In order to procure sugar 

 or starch, we must supply the plant with other materials than 

 we would do were our object to obtain the ingredients containing 

 sulphur and nitrogen. 



In a hot summer, when the deficiency of moisture prevents 

 the absorption of alkalies, we observe the leaves of the lime-tree, 

 and of other trees, covered with a thick liquid containing a large 

 quantity of sugar ; the carbon of this sugar must, without doubt, 

 be obtained from the carbonic acid of the air. The generation 

 of the sugar takes place in the leaves ; and all the constituents 

 of the leaves, including the alkalies and alkaine earths, must 

 participate in effecting its formation. Sugar does not exude from' 

 the leaves in moist seasons; and this leads us to conjecture, that. 



