22 MERISTEa^IS 



-what Schmidt (1924) called "minimal" and "maximal" areas 

 (cf. also Louis, 1935, PI. II, Figs. 20-21; and Cross, 1937, Figs. 

 10-11). This situation emphasizes the fact that the form and 

 the dimensions of the shoot apex are likely to vary depending 

 upon whether an active or dormant apex is measured as well as 

 upon the particular phase in shoot development which is under 

 examination. Extremely few careful measurements have been 

 made of the shoot apex of seed plants, and no generalizations are 

 possible at present. Apparently, however, the angiosperms typi- 

 cally possess rather small apices which range in diameter from 

 90^ in certain grasses to 500,a in some of the palms. Possibly 

 130-200/1 may prove to represent a frequent range in diameter 

 of the apex in dicotyledons (Boke, 1940). The width of the 

 shoot apex of some conifers, of Ginl-go hiloha, and of Zamia 

 slightly exceeds that of "typical" angiosperms. But in the Sago 

 Palm {Cycas revoluta Thunb.), the shoot apex may attain the 

 relatively enormous diameter of 3.5 millimeters, a dimension 

 gi'eatly exceeding that recorded for any vascular ])laiit (cf. Fos- 

 ter, 1940). The nature of the relationship between size and form 

 of the apex, on the one hand, and the morphology and primary 

 anatomy of the shoot, on the other, is obviously complex and 

 awaits further comparative studies for its solution (Bower, ]9:)0, 

 Ch. XII; Foster, 1939b, 1940, 1941a, 1941b). 



When a thin median longi-section of the shoot apex of (di 

 angiosperni is examined under the microscope, two pi'incipal 

 zones or regions are usually distinguishable, viz.: (1) the tunica, 

 which consists of one or more discrete superficial layers of cells, 

 and (2) the corpus, which is a "core" occupying the center of 

 the apex and exhibiting an irregular or "i-andom" arrangement 

 of cells.' The differences in cell arrangement in tunica and corpus 



^ In both of these zones, the cells are relatively siiiall and in sectional 

 view appear " isodianietric " in form. Jjittle is known, iiowever, al)out tlie 

 shape of such cells when regarded as three-dimensional structures. Ac- 

 cording to I'riestley and Scott (li)3S pp. 201-L'Oi2) macerated cells of the 

 apex "appear as rather irregular, many-sided figures, the facets of which 

 are mainly hexagonal or s(|iiare. " By compressing sidieres of plasticine, 

 these investigators ohtained 12-sided liodics wliidi thi'y assume are simi- 

 lar in form to nicristem cells. They conclude that "the shape of the cells 

 is tlius exjdained as the natural result of the growth and division of plas- 

 tic hodics miller mutual pressure." (For further information on the prob- 

 lem of form in isoilianu't ric plant cells cf. Kx. \'l, i)p. 57-.i8. 



