100 REPRODUCTION OF PLANTS. 



phenomenon exists in many other plants under 

 various forms. In the cases in which vege- 

 tation commences in the descending system, 

 that is, in which roots, whose development is 

 always effected through the descending juices, 

 are first formed, the result is produced in some 

 portion of the stem which is found to contain a 

 deposit of nutritive matter, and which is within 

 reach of moisture. This effect occurs naturally 

 in some stems, but is facilitated by any cause 

 which tends to arrest the nutritive juice in its 

 descent, and so to form an accumulation of it at 

 a given part. Thus in nature when a portion 

 of a stem containing such an accumulation, is 

 buried beneath a humid soil, and has a fleshy 

 bark, it tends to put out roots, which it does 

 naturally by what are called "suckers," and 

 man, profiting by this provision, adopts the me- 

 thod of increasing by layers, pipings, cuttings, 

 &c. since it is found that the part thus endowed 

 may be separated from the parent trunk, and 

 being composed of the two parts that constitute 

 an individual plant, a stem and a root, is capable 

 of an independent existence. In some instances 

 a leaf planted in the ground will vegetate from 

 its central nervure. 



74. There is one great difference notwith- 



