68 Growth 



plant grows. At the onset of reproductive maturity or at the end of the 

 life cycle it often becomes reduced again in size. In maize, a plant 

 essentially determinate in its growth, Abbe and his colleagues (1951a, 

 1951b, 1954) have studied the size of the apical shoot meristem (above 

 the first leaf primordia) and the size and number of its cells as these 

 change with time. In plastochrons 7 to 14 (the seedling stage until just 

 before flowering) the apex increases by a constant amount in each 

 plastochron but the duration of the plastochron decreases exponentially, 

 from a length of 4.7 days to one of 0.5 day. Cell size is essentially con- 

 stant throughout, so that all growth is by cell multiplication. The growth 

 rate per plastochron accelerates exponentially. In the five or six plasto- 

 chrons during early embryogeny, on the contrary, the duration of suc- 

 cessive plastochrons increases and the growth rate of the apex decreases. 



Sunderland and Brown (1956) have determined the cell number and 

 average cell volume in the meristematic dome and the first seven 

 primordia and internodes, back from the apex, in the shoot of Lupinus. 

 The primordia increase exponentially in successive plastochrons but there 

 is little increase in cell volume in the internodes. 



Cell shape in the shoot apex of Anacharis (Elodea) has been studied 

 by Matzke and Duffy (1955) with particular reference to the number 

 of faces. These range from 9 to 21 and are in general agreement with 

 the shape of cells in other undifferentiated tissues. 



For a statement of conditions in the apical meristem of the shoot in 

 ferns the reader is referred to Wardlaw (1945). This author has also 

 published an extensive series of papers on experimental and analytical 

 studies of pteridophytes, many of which are cited in his books (1952«, 

 1952&) and in later papers by himself and his colleagues. The shoot 

 meristems of gymnosperms are described by Camefort (1956) and John- 

 son ( 1951 ) . General accounts of this region in the angiosperms, with re- 

 views of the literature, have been written by Foster ( 1939, 1949 ) , Sif ton 

 (1944), Philipson (1949, 1954), Popham (1951), Buvat (1952), and 

 Gifford (1954). 



The ontogeny of a typical shoot apex (Xanthium) has been described 

 in detail by Millington and Fisk (1956). 



EXPERIMENTAL STUDIES ON THE SHOOT APEX 



Early work on the shoot apex was primarily descriptive, and much of 

 this still continues. It has been concerned with apical cells, planes of 

 division, cell lineages, layering, zonation, and the relations of the meri- 

 stem to differentiation and organ formation. This work has been of great 

 value morphogenetically for it has provided a fund of information as to 

 the structure and developmental activity of this determinative region 



