170 



being no reserves in the tissues of the stump and roots avail- 

 able for the production of such shoots. If the severed living 

 tissues on the lower edge of the ring receive sufficient 

 nourishment from leafy shoots situated below the ring, they 

 will also proceed to form a callus, which may meet and coalesce 

 with that developed from the upper lip, the wound being thus 

 healed over and the normal continuity of the tissues re-estab- 

 lished, although, here again, the wider the ring, the longer 

 will the healing be delayed and the more injurious will be the 

 wound. Cases will be occasionally met with which do not 

 appear to admit of an explanation according to the above 

 facts, and such exceptions will be found due to the existence of 

 special conducting tissue in the interior of the stem, wood which 

 has retained its power of conductivity, internal phloem, etc. 

 As a general rule, however, the non-success of girdling is due 

 to the operation not having been thoroughly performed, 

 complete interruption of the ordinary conducting channels not 

 having been secured. In some cases it is possible that the failure 

 of the operation is due to the roots of the girdled tree being in 

 intimate connection with the roots of neighbouring trees, and to 

 their thus being able to obtain their necessary food materials 

 from such trees. 



-. . 148. Now the callus which, as 



we have seen above, so frequently results from a wound, 

 consists, at all events in the early stages of its development, 

 largely of embryonic cells, i.e., cells which must have the 

 capacity of producing any part of the plant body in which they 

 occur, for the whole plant has been built up by the activity 

 of such cells. Hence it is not surprising that the callus is often 

 found to give rise to roots, and buds capable of developing into 

 shoots. Such roots and buds are termed adventitious, as are all 

 structures which occur in places where under normal conditions 

 they would not have been developed. This may be well seen 

 in the case of cuttings, these being shoots which have been 

 separated from the parent plant by a clean knife cut. The base 

 of such a cutting, if kept moist by being placed in soil or damp 

 air, soon becomes covered with a callus developed from the 

 living growing tissues adjoining the cut surface, in the manner 

 described above. Roots then spring from this callus and through 

 the cortex at the base of the cutting, the latter thus becoming 

 an independent plant. It is important to restrict evaporation 

 from the cutting as much as possible until the new roots have 

 been developed, and the cuttings are often first laid sloping- 

 wise in the ground and entirelv covered with earth with the 



