36.] CHAPTER II. THE TISSUES. 215 



Stems of plants of the same species will grow together if they 

 are in close contact ; the callus formed by the cortex of both, 

 coalesces and gives rise to a common cambium. On this depend 

 the various modes of artificial grafting, in which branches or bnds 

 with a portion of the cortex are taken from a variety or an allied 

 species and placed so that their cambium is in contact with that 

 of a stem which serves as the stock, and subsequently they grow 

 together. 



In conclusion, the mechanism by which deciduous members (see 

 p. 22) are detached has to be considered : the fall of the foliage- 

 leaf may be taken as the illustration. In some cases {e.g. Palms; 

 some Ferns, as in the section Phegopteris, p. 405 ; the Oak) the 

 leaves simply wither on the stem, when they are non- articulated, 

 and are gradually destroyed and removed; but in most cases they 

 are thrown off by a vital act before they wither, when they are 

 articulated. The fall of the articulated leaf depends upon the 

 growth and division of all the living cells lying in a transverse 

 layer near its insertion: by this means several (3-6) layers of 

 compact tissue are formed. A median layer of this tissue becomes 

 disorganised, and then the leaf is held in position only by the 

 vascular tissue which enters it from the stem. This soon breaks 

 under the weight of the lamina, especially if it be agitated by the 

 wind, and the leaf falls. The disorganisation of the median layer 

 is often accelerated by the action of frost. The scar on the stem 

 (leaf -scar, p. 23) either simply dries up ; or a layer of cork is 

 formed over it by the merismatic tissue which remains : in any 

 case the vessels become sealed with mucilage. 



