Meristems 73 



ferent stages in its development. Something is thus being learned about 

 the chemical as well as the histological organization of the shoot apex. 



The techniques of tissue culture have also proved to be very useful for 

 a knowledge of meristem physiology. It has been found (Wetmore, 

 1954), for example, that the shoot apex of vascular cryptogams, when cul- 

 tured with only inorganic substances and sucrose, will produce entire 

 plants but that they will grow better if supplied with auxin and some 

 nitrogen source other than nitrates. Angiosperm apices (Syringa), how- 

 ever, will not grow in this simple medium, but if coconut milk and casein 

 hydrolysate are added, the tips root, and growth is much better. When in 

 the culture medium the same amino acids and amides are provided, and 

 in the same proportions, as are found in meristem tissue, growth is still 

 very slow and far from normal. Evidently something more is necessary. 



Wetmore's demonstration (1954) that when apices of sporeling ferns 

 are cultured with successively higher concentrations of sucrose the leaves 

 that they produce correspond to those formed in progressive stages of 

 normal ontogeny (p. 222) shows the important morphogenetic and mor- 

 phological implications of nutritional factors. 



The rates of metabolic processes in the shoot apex have also been in- 

 vestigated. Ball and Boell (1944), using the Cartesian-diver technique 

 by which it is possible to measure the rate of respiration in tiny bits of 

 living tissue, compared this rate in the apical dome of cells, the region 

 just below this where the first primordia are appearing, and a third region 

 below this (Fig. 4-15). In Lupinus, respiration was most active at the tip 

 and progressively less so below. In Tropoeolum, however, there was less 

 respiration in the extreme tip than in the region below it. The occurrence 

 of a descending metabolic gradient in the plant apex thus seems not to 

 be universal. 



These various experimental studies on the shoot meristem have directed 

 attention even more strongly to this embryonic region. Many believe that 

 it is of primary importance for development and that in it the major 

 problems of morphogenesis, at least as far as the shoot system of vascular 

 plants is concerned, come sharply to focus. This conclusion is supported 

 by the facts that the apex is autonomous, a small portion of the tip of it 

 being able to produce an entire plant; and that if it is removed, the 

 development of the tissues below it finally stops. It is recognized, how- 

 ever, that certain structures, such as leaf primordia beyond a certain 

 stage, are partially removed from its control since they will develop in- 

 dependently in culture and are thus self-differentiating. 



Ball has compared the shoot apex to an organizer such as has been 

 postulated in animal embryology. Such, in a sense, it is, but the com- 

 parison is not very exact since the organizer is a part of the embryo 

 which controls the development of the rest, wheeras the shoot apex 



