238 The Phenomena of Morphogenesis 



been done by Ball in Lupinus and Tropaeolum and by Wardlaw in 

 Primula and several ferns. Ball ( 1948, 1952« ) went still further than 

 Pilkington and was able to split the meristematic apex into four, and later 

 even into six, strips ( each still connected basally with the axis ) and found 

 that each was able to reconstitute a whole shoot unless its tip had been 

 reduced below a minimal size. Vascular tissue tended to be poorly devel- 

 oped in it until leaves were formed by the new shoot. 



Both Ball ( 1952/? ) and Wardlaw ( 1950 ) isolated the central core of the 

 shoot meristem from the rest of the axis by three or four longitudinal in- 

 cisions (Fig. 4-14), thus leaving the meristematic dome (or the apical 

 cell and its neighbors) connected with the vascular tissues below only 

 by a plug of pith tissue. Both investigators found that this isolated tip 

 continued to grow and in time produced procambial tissue independent 

 of that in the axis below. In the flowering plants studied this differen- 

 tiated basipetally and finally joined the vascular system of the main axis. 

 In the ferns, however, it did not do so. At the apex of the isolated core 

 new primordia were formed and (except in ferns) if this core was not 

 below a minimal size, it finally developed into a normal leafv shoot. 

 Wardlaw observed that the phyllotaxy of the new shoot in Primula was 

 continuous with that of the original axis but in Ball's material it was inde- 

 pendent and often showed reversal of the earlier spiral. Wardlaw ob- 

 served that the vascular tissue developing in the central core followed the 

 outline of the cut piece and differentiated at a rather constant distance 

 from the cut surface, suggesting that its position was dependent on a 

 gradient of some sort (Fig. 9-2). 



These experiments are of morphogenetic interest since they show that 

 the apical meristem is a self-determining region which can produce a nor- 

 mal shoot without any connection, other than through undifferentiated 

 pith cells, with the tissues below. Furthermore, Ball (1946) and others 

 have shown that small meristem tips, growing in tissue culture, will pro- 

 duce entire plants. All this is not surprising, however, since many cases 

 are known where a single cell (p. 253), without any vascular connection 

 with other tissues at first, develops into a fully differentiated plant. It is 

 to be expected that an active terminal meristem would do the same. Al- 

 though the differentiated tissues below the meristem may not be necessary 

 for its growth, they contribute to the character of its development, for 

 through them come water, nutrients, and various morphogenetically im- 

 portant substances. The terminal meristem seems neither to be completely 

 self-determining nor completely under the control of the rest of the plant, 

 but the two act together as an integrated system. 



Reconstitution of parts other than the meristems of the axis has often 

 been reported. In a number of ferns such as Gleichenia, the leaf grows 

 at the tip by a terminal meristem, thus perhaps harking back to the time 





