222 The Phenomena of Morphogenesis 



ological character. Rietsma, Satina, and Blakeslee (1953a), by growing 

 Datura embrvos in tissue culture, have shown that the minimal sucrose 

 requirement falls steadily from the earliest stages to the mature em- 

 bryo. 



A notable example of physiological ontogeny has been reported by Wet- 

 more (1954). In the developing fern sporeling the first leaves are two- 

 lobed. These are followed by three-lobed ones and finally by pinnate 

 leaves in which an apical cell has appeared. Shoot apices from small fern 

 sporelings, cultured in mineral nutrients and various concentrations of 

 sucrose, grew into whole plants. Where the concentration was low, only 

 two-lobed leaves were formed. Higher concentrations produced three- 

 lobed ones and still higher, pinnate ones. The normal ontogenetic progres- 

 sion here thus seems to be related to an increasing supply of sucrose. 



Metabolic gradients are marked by various physiological differences, 

 especially as to the rates of reactions. Prevot ( 1940) observed that respira- 

 tion in the apical region of the root of several genera was greater than 

 in the more distal regions. This is not always the case, however, in shoot 

 meristems (p. 73). 



Wardlaw (1952c) has found that the nutritional status of the apical 

 region in ferns has an important effect on the size and character of the 

 leaves and stelar structure. Apices that normally produce large and com- 

 plex leaves and an elaborate vascular system, if reduced in size by poor 

 nutrition, will form "juvenile" leaves and simpler stelar patterns. 



Biochemical differences of many sorts, presumably indicating physi- 

 ological diversity, can be shown in cells and tissues. Differential staining 

 reactions are familiar examples of this. Differences in hydrogen-ion con- 

 centration between cells visibly alike can be shown by the use of indi- 

 cators. Blakeslee (1921) demonstrated the presence of two chemically 

 different areas of cells in the petals of certain races of Rudbeckia with 

 solid petal color by dipping the petals into phenolphthalein. Sometimes 

 differentiation is shown by the occurrence of natural pigments, as in the 

 root tips of Phalaris arundinacea where the trichoblasts are pigmented 

 but the cells that are not to produce root hairs are colorless. Van Fleet's 

 work on the histochemical differentiation of the developing endodermis 

 has been mentioned (p. 197). Microchemical tests of various sorts being 

 out differences between many kinds of cells, even in early development, 

 such as tannin cells, crystal cells, and latex ducts. Spectrometric demon- 

 stration of differences in distribution of the nucleic acids are among 

 notable recent examples of this sort of analysis. 



Less work has been done in demonstrating physical differences be- 

 tween cells. In fern prothallia Akdik (1938) and Gratzy-Wardengg (p. 

 121 ) found a definite pattern of differences in osmotic concentration over 

 the surface of the prothallus, and this seems to be related to differences in 



