THE ORGANS OF PLANTS. 57 



so, when it is remembered that the layer is so thin that it cannot be removed without 

 detaching also a portion of the vegetable tissue. 



Proportion of Silica, or flint, in 1000 parts of the ashes left after burning the following 



vegetable substances. 



Oat straw . . . . .45- 



Barley ...... 38-5 



Wheat ...... 287 



Indian corn . . . . .27* 



Oak leaves . . . . .15' 



Ferns ...... 10'4 



Pea straw . . . . .10' 



Potato tops . . . . 8' 



Heath . . . . . 5'S 



Beans ...... 2'2 



Bean straw . . . . .2* 



Cabbage . . . . .2*1 



Buckwheat . . . . .1-0 



This subject has an important bearing upon the rotation of crops, for it is manifest 

 that if successive crops of corn, and especially of oats, be obtained from the same land, 

 there must be an enormous expenditure of this necessary article ; but that a much less 

 quantity suffices, if potatoes, pease, beans, or cabbage be given as intermediate crops. 

 So, also, with regard to manures. It is clear that a manure must not only contain the 

 carbon which forms the straw, and the salts which are always found with it, but there 

 must be a constant and abundant supply of silica. This is effected by using corn, and 

 especially oat straw, as manure, and also by the use of guano, which contains a large 

 per centage of silicious skeletons. 



THE ORGANS OP PLANTS. 



Having now considered, in such detail as our space has permitted, the various ele- 

 mentary tissues which have been discovered in vegetables, and the juices and secretions 

 which they contain, we proceed to describe the parts or organs which are formed by 

 their combination. Such are the leaves, flowers, and fruit, and the structures which 

 support them. 



The modes in which we might proceed are numerous, and partly arbitrary, varying 

 with the fancy of each author ; for no one arrangement of the organs of plants is found 

 in Nature which is acknowledged by all investigators to be more natural than any 

 other. 



The nearest approach to Nature will be found in proceeding either centripetally or 

 centrifugally : that is, either first to describe the seed, and thence pass to the centre of 

 the stem, through the fruit, flowers, leaves, and other appendages to the stem : or to 

 commence at the stem and roots, and then clothe these organs with leaves, flowers, and 

 fruit, in the order which nature has selected. Of these two we prefer the latter 

 course, and shall proceed to describe the stem, with its root, and the various organs 

 supported by them. 



