20 ■ MERIRTEMS 



exhibiting vacuolating cell growtli and division." Priestley's 

 effort to demarcate meristems on the basis of the absence or lack 

 of prominence of vacuoles is not supported by comparative 

 studies. For example, the apical cell and its most recent segments 

 in many lower vascular plants are highly vacuolate in character 

 and it is only at some distance from the summit of the apex that 

 small "dense" cells are found to occur (Eames and MacDaniels, 

 1925, Fig. 28; and Zirkle, 1932, taf III, Fig. 16). Likewise, in 

 the shoot apices of Ginkgo hiloha and Cycos rcvoluta, Zamia, and 

 Dioon cdule a more or less w'ell-defined central group of enlarg- 

 ing vacuolated cells, surrounded by smaller and more densely- 

 cytoplasmic cells is present (cf. Foster, 1938, 1939a, 1940, 1941b). 

 These examples clearly show that the relative position and extent 

 of "zones" characterized by the predominance of cell division or 

 cell enlargement are variable in the shoot apex of vascular plants 

 (Foster, 1941a; Boke, 1941). In short, cell division and con- 

 spicuous vacuolation are not processes confined in Priestley's 

 sense respectively to the summit and lower portion of a shoot 

 apex. On the contrary, these processes may overlap at the same 

 level in a growing apex. Doubtless the most significant evidence 

 of the vacuolated character of meristems has been secured by 

 recent studies on living tissue. The work of Bailey (1930) and 

 Zirkle (1932) on the vascular cambium and primary meristems 

 respectively has indicated that all meristems are vacuolated, and 

 furthermore, that the form of the "vacuome" varies within wide 

 limits at different seasons of the year and at dif!'ereiit stages of 

 growth ill the same type of meristem. 



From the preceding brief critique it should be evident that 

 it is imj)ossible in the light of present knowledge to frame an 

 adequate "definition" of meristematic tissue. On the contrary, 

 it seems increasingly clear as investigation jiroceeds, that we have 

 to deal with various and i)ossibIy distinct types of "meristem," 

 at least from a i)liysiological viewpoint. How the organization 

 and growth of meristems is related to the orderly progressive 

 differentiation of tissues from apical or latcM-iJ meristems consti- 

 tutes one of the most challenging prol)lems in Tiiodcrn botany. 

 Further insight will come when tlio results of comj^arative obser- 

 vation ai-c checked by experimental studies. It seems likelv that 



