1922-23.] BOTANICAL SOCIETY OF EDINBURGH 18T 



adjacent to the xylem are living and are in an active state of 

 division, the divisions being laid down at right angles to the 

 wounded surface, differing in this respect from those described 

 by Sorauer (3), which are laid down parallel to the cut surface. 



This development corresponds with that described by Stoll (4) 

 for Hibiscus Reginae, Passiflora quadrangular is, and Giselina 

 littoralis, viz. " That every tissue of the plant excepting true 

 wood, bast and epidermal cells, is capable of aiding in the 

 construction of callus, the initial and chief growth being from 

 the cambium." Cell division was also found in the pericycle 

 and inner cortex as described above. As described by fSimon (5) 

 for Populus, spp., it is impossible here also to separate cambial 

 and cortical callus, because it is formed simultaneously by 

 divisions parallel to the wounded surface. The activity of 

 the medullary meristcm, coupled with the growth from the 

 cambial, pericyclic, and cortical meristems, results in the 

 closing of the wounded surface. 



Important questions arising out of this investigation are, 

 firstly, what has become of this pericyclic stereom during 

 etiolation ? and, secondly, why should callus formation proceed 

 more easily in its absence ? There is no reason to suppose 

 that the process of lignification is irreversible as long as the 

 cell in question remains alive. The answer to the first question 

 then depends on what is the acknowledged function of lignified 

 tissue. Is it purely mechanical, or may it not act also as a 

 form of reserve food, accumulated and laid down only under 

 tonic conditions ? If so, the problem is simplified, for naturally 

 during the process of etiolation the plant would have to depend 

 on its internal reserves. Regarding the second question, it 

 is recognised that any undifferentiated tissue capable of rapid 

 divisions may become meristematic, under given conditions, 

 and form callus, and as cell division is evident in the pericycle, 

 this tissue is able to contribute when in a non-lignified con- 

 dition. It seems reasonable to suppose then that callus 

 formation would proceed more easily in the absence of a 

 lignified pericycle, which might present a formidable barrier 

 to movement of reserve, and to growth in thickness, as the 

 chief growth of callus takes place in the cambium. 



This paper in no way aspires to deal fully with a subject 

 in which there are so many possibilities. The effect of etiola- 

 tion on the natural oils, on the translocation of food, etc.^ 



