30 



BUDS ON STEMS. 



is quite covered over by the new growth. The latter is termed "callus", and may 

 be compared to the tissue which is developed when an arm or a foot is amputated, 

 and which grows from the ligaments beneath the skin until it gradually covers the 

 whole stump. The callus in plants derives a special interest from the fact that 

 within it are formed the rudiments of fresh buds, from which subsequently spring 

 the shoots which "break" so plentifully. A longitudinal section through an Oak 

 stump thus overgrown shows the callus wedged, as it were, between the old bast 

 and the old wood; and we find that it consists of cork and parenchymatous cells, 

 whilst vascular tissues, springing from the wedged portion of the callus, have 

 also been developed, and, descending in bent and tortuous lines, establish an organic 

 connection with the old trunk. The buds arising in the callus do not stand in any 

 relation of any sort to the leaves, as has already been mentioned; nor do the 

 intervals between them follow a geometric law, as is the case with the buds which 

 take their rise from the axils of leaves. They are for the most part in aggregations 

 and are produced anything but simultaneously. A callus of the kind may con- 

 tinue to produce buds at appropriate spots year after year, and shoots of many 

 different ages may be seen springing from it. One cannot contemplate such a 

 callus growth, covering a stump and sending out shoots as direct off-shoots of the 

 decapitated trunk, without being involuntarily reminded of trees that have been 

 " ennobled " by grafting in the manner described in vol. i. pp. 213, 214. There is 

 also an analogy to certain parasitic plants, such as Loranthus, in which the 

 connection with the host is established in exactly the same way as that between 

 callus-buds and tree-stump by means of a tissue interposed between wood and bark 

 (c/. vol. i. p. 211). 



A formation of callus ensues upon the excision of the cortex from the side of a 

 stem in the same manner as when the entire trunk is sawn through; and the 

 process of covering up the exposed wood with callus, derived from the tissue lying 

 between the bark and the wood, goes on similarly in the case of lateral injuries to 

 the trunk. Some trees in addition exhibit a formation of callus without external 

 damage having been received, as, for instance, the Ash, which has a bark liable to 

 split and break open here and there spontaneously, whereupon a tissue of the nature 

 of callus is formed in the open places. Oldish trunks of the North- American Ash 

 (Fraxinus nana) are invariably covered with swellings and callosities of the kind, 

 and most of them furnish starting-points for a score or more of buds. 



The buds which spring from growths of callus on trunks must not be confoimded 

 with those called by foresters "dormant eyes" and "dormant buds''. Nor must 

 we fail to distinguish them from the structures which have been termed superposed 

 and collateral buds, which whilst exhibiting extreme diversity in their various 

 modes of development, yet all constitute contrivances for the preservation of the 

 plants from destruction in that their function is to replace dead shoots. With 

 reference to the part played by these structures, it is most convenient to classify 

 them under the name of "reserve-buds". They either originate simultaneously 

 with those which they are destined in certain circumstances to replace, or they 



