156 PART II. ANATOMY AND HISTOLOGY. [ 36 



may easily be found. A similar explanation accounts for the fact 

 that the surfaces of the stumps of cut-off branches become over- 

 grown : the callus first appears as a ring from the cambium ex- 

 posed in the transverse section, and afterwards closes like a cap 

 over the old wood (Fig. 121). Foreign bodies nails, stones, and 

 stems of other plants may thus become enclosed in the wood of 

 a tree and be over-grown by it ; the cortex, being forced against 

 the foreign object by the pressure of the growing wood, splits, and 

 the callus formed in the rent grows round the object, enclosing it 

 and producing new cambium. 



Steins 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 orjbuds 

 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. 10). 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 ; 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 said 

 to be articulated. The fall of the articulated leaf depends upon 

 the growth and division of all the living cells lying in a trans- 

 verse layer near its insertion ; by this means several (3 6) layers 

 of compact tissue are formed. A median layer of this tissue, the 

 abscission-layer, 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 disorgan- 

 isation of the median layer is often accelerated by the action of 

 -frost. The scar on the stem (leaf-scar, p. 10) 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 mu- 

 cilage. 



