Mason — Note on Growth of Bitter Cassava. 109 



The Transport of Organic Substances. 



Inspection of Table 2, in which are recorded the weights of the tuberous 

 portions of the roots, and of tlie three segments of the stem, will show that the 

 subterranean storage organs were more than four times as heavy in the unringed 

 as in the ringed plants. From this, and the greater weight of the basal portions 

 of the stem in the latter group (cf. Table 2), it would seem that the transport of 

 organic substances, especially carbohydrates, must have ceased, eitlier completely 

 or almost so, from the time of ringing ; many of the tuberous roots of the ringed 

 plants were, in fact, obviously shrunken. It cannot, of course, be inferred from 

 this that the translocation of carbohydrates occurs in the phloem rather than in 

 xylem.' When, however, one considers the results in the light of the evidence 

 adduced by Dixon and Ball (2) in support of the view that the xylem is the 

 channel for the longitudinal movement of carbohydrates, it would seem that though 

 the actual translocation may take place in the xylem, yet the phloem must take 

 an active part in this transmission ; from the view-point of correlation such a 

 hypothesis assumes a not improbable aspect. 



It will be patent that the movement of organic substances in the plant, now 

 in one direction and now in another, according to the phase of development, 

 must in some way be dependent on the mechanism which correlates the varied 

 activities of the oi'ganism (cf. Smith (8)). It is a matter of counnon knowledge that 

 the removal of the growing apex of a shoot in some way releases the buds 

 immediately below from their condition of dormancy. The state of dominance and 

 subordination is sometimes (cf. Eeed and Halma (4)) ascribed to the transport of 

 specific substances (hormones), and sometimes (cf. Child (1)) to the transmission 

 of an excitation through the living protoplasm. In the course of the work reported 

 here, it was observed that, shortly after the removal of the ring of extra-xyliary 

 tissues, the bud immediately below the ring commenced to grow.- 



It would seem that the removal of the ring of extra-xyliary tissue not only 

 interrupted the transport of carbohydrates, but also blocked the transmission of 

 the correlating agency, whatever its nature, which is responsible for the condition 

 of dormancy in the lateral buds. Now, if it be admitted that the movement of 

 organic substances in the plant is in some way dependent on the existence of this 

 correlation factor, and if it be further granted that the presence of the phloem is 

 necessary for the transmission of this factor, it ought to follow that, though the 

 actual channel for the movement of carbohydrates may be the xylem, yet the 

 removal of a ring of phloem would interrupt their translocation. 



The Internal Factors Controlling Growth. 



It is now possible to consider what are the internal factors which determine the 

 changes in the activity of the cells of the apical meristem. In what follows it will 

 be presumed that the changes observed in the rate at which the stem elongated 

 were determined by similar changes in the activity of these cells. If it be assumed 

 that their activity was controlled by the supply of organic substances available, 



' It will be evident that the downward flow of carbohydrates in the ringed plants cannot 

 have ceased as a result of tlie blocking of the tracheae ; for, had this occurred, the ascent of 

 water and the inorganic solutes necessary for growth would have been similarly checked. 

 Nor, it may be remarked, does an examination of the xylem in the region of the ring afford 

 any grounds for such a view. 



" These shoots wei-e removed after they had attained a length of a couple of centimetres. 



