Chemical structure and biological activity 



Caplin, and Shantz, 1955, Plate I). As Figure 3 shows, it often happens that 

 after about 10 days the average cell size begins to change in a way that 

 clearly indicates that vacuolation and growth by cell enlargement now 

 makes an appreciable contribution to the total growth of the tissue system. 

 In this system it is found that in the most rapid phase of growth the cell 

 divisions must occur so that successive cell layers are formed approximately 

 every 4 hours. This rapid rate is, however, not inconceivable, and it is quite 

 clear from the large number of cells produced in these cultures that divisions 

 must occur with this order of rapidity. 



(b) Ai^erage cell size/ lime 



8 12 ie ^0 



Time days 



8 12 IS 20 



Time days 



Figure 3. Growth of carrot explants in medium containing coco-nut milk. 



It is, however, abundantly clear that the growth surrounding small 

 inocula, producing successive layers at regular intervals of time, cannot 

 proceed indefinitely or even for very long. Vacuolation and the formation of 

 large air spaces certainly occurs in the larger cultures. It may be seen from 

 Figure 3 that in the example to which this relates, growth by cell enlargement 

 also became evident after 10 days. Visual examination of the anatomy of the 

 cultures clearly shows that as they become larger the familiar local centres 

 of growth, or 'buds', appear on the surface as minute proliferations. Around 

 the familiar lignified elements in these tissue cultures, local centres of growth 

 organize that eventually may lead to roots [Figure l{b), (c), and (d)). Thus, 

 although the concept indicated above is clearly useful for the analysis of the 

 earlier stages of growth, it is quite clear that it cannot be projected into the 

 later periods, in which some parts of the culture may still indulge in random 

 proliferation, while others are growing in a more organized fashion. Most of 

 the work now to be described will, however, have concentrated upon the 

 more early stages in the growth that is capable of more exact analysis in 

 terms of cell number and of cell size. 



CHEMICAL FACTORS IN GROWTH INDUCTION 



The analysis, by these various means, of chemical factors involved in the 

 induction and regulation of growth in these tissue cultures can best be 

 developed in two parts. 



172 



