Meristems 71 



(through its place in the phyllotactic series) and then isolated it from 

 the apical cell by a deep tangential cut. Under these conditions a bud 

 rather than a leaf primordium developed. Evidently some influence com- 

 ing from the apex determines whether a new lateral outgrowth will form 

 the dorsiventral primordium of a leaf or the radially symmetrical, po- 

 tentially indeterminate one of a bud, a discovery of much importance 

 not only morphogenetically but morphologically. Cutter (1956) has 

 found that the three youngest primordia respond in the same way but 

 that older ones do not. These same young primordial areas (in Osmunda) 

 if excised and grown in culture will form buds and finally mature 

 plants, whereas the older areas, if cultured, will grow into typical leaves 

 (Steeves and Sussex, 1957). There is evidently a point before which the 

 lateral structure has the potentiality to form either a bud or a leaf, but 

 after a certain early stage has been reached its fate is determined. 



Wardlaw has extended these studies further (1956a, b). When in- 

 cisions between apex and primordium were so shallow that the pre- 

 vascular strands were not cut, a leaf primordium still developed, sug- 

 gesting that this incipient vascular tissue is a pathway for morphogenetic 

 stimuli. When deep cuts were made on the radial and obaxial sides of a 

 primordium site, thus without isolating it from the apex, a leaf grew from 

 it, but this usually showed abnormally rapid growth. 



In flowering plants results like these were not obtained, for isolated 

 primordia do not develop into buds but into dorsiventral leaves or 

 radially symmetrical leaf-like structures (Sussex, 1955). However, Cutter 

 (1958) found that in Nymphaea and Nuphar (favorable material be- 

 cause of their large meristems), although tangential cuts separating a 

 primordium from the apex did not change it to a bud, buds under these 

 conditions were formed more often and very close to the primordium. 

 The critical time for the determination of the fate of a primordium thus 

 seems to be earlier in flowering plants than in ferns. 



The problem of phyllotaxy is closely related to conditions at the meri- 

 stem, for the arrangement of the leaves is presumably determined by 

 the distribution of their primordia. This subject will be discussed more 

 fully in the chapter on Symmetry, but it should be mentioned here that 

 the experimental work of the Snows and of Wardlaw on the factors that 

 determine where a given primordium shall arise has yielded much in- 

 formation. It is evident that the apex has an important influence on the 

 differentiation of primordia, but whether the exact determination of the 

 position of these structures results from chemical, mechanical, or geomet- 

 rical factors is still not clear. Experimental manipulation of the meristem 

 is a hopeful way of approaching this problem directly. 



The differentiation of vascular tissue seems to depend on a stimulus, 

 probably a growth substance, passing basipetally from the apex. The 



