30 BUnS ON STEMS. 



is quite t'ovoroil over by the now i^rowiili. Tlio Inltor is (ci'iued "callus", mui may 

 bo compared to the tis.suo wliicli i« ilin(^lo)iO(l wIumi iin nriw or a I'oot is amputiitod, 

 tiud which grows t'roni the ligaments boni'n.th |.lu> skin until it frraduJilly covers the 

 whole stump. The callus in plants deriv(>s a speeiiil inter(>st I'roni (he i'act that 

 within it are formed the rudiments of fresh buds, from whieh subsequently spriuL;- 

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

 stump thus overgrown shows the callus wedgeti, 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 wedg(>(l portion ol' the callus, ha\e 

 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 tluur rise from the axils cil' leaves. They are for the most part in aggregations 

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

 tinue to produce buds at appropriate spots year after year, and shool-s of many 

 (liU'eroiit ages may be seen springing from it. One cannot contemplate such a 

 callus growth, covering a stump and sending out shoots as <lii-ect olF-shoots of the 

 decapitated trunk, without being involuntarily reminded oi trees tha,t have bet^i 

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

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

 connection with the host is established in exactly the sM,me way as that between 

 callus-buds and tree-stump by means of a tissue int(!i-])os(Kl betweeni 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 IVom the tissue lying 

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

 the trunk. Some trees in fuldition 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 lien^ 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, 

 ari<l 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 confounded 

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

 we fail to distinguish them from the structures which have 1 loen termed superposei I 

 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 noplace dead slioots. With 

 reference to the part played by these structures, it i.s most convenient to classify 

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

 with those which they are destined in certain circumstances to replace, or th(^y 



