12 EMBRYOGENESIS IN PLANTS 



will lead to orthogenetic evolution. As Goldschmidt (1927) has pointed 

 out, the sooner a gene begins to act, the more it can do. 



According to Goldschmidt (1938), either the genes or their immedi- 

 ate products act as catalysts. Because an enzyme is specific in its action, 

 it will only act if the proper substratum and suitable sustaining con- 

 ditions are present. When all the conditions are satisfied, the action of 

 the enzyme, which might have been present in the zygote since fertilisa- 

 tion, though perhaps not in sufliicient concentration, may begin. This 

 is described as the activation of the gene, and, from the metabolic 

 standpoint, embryogenesis may be regarded as a sequence of cyto- 

 plasmic states, each of which is initiated under genie control. Thus, as 

 the zygote grows, the original cytoplasmic substratum is transformed 

 into two or more different ones, these providing the substrates suitable 

 for the action of new genes. 



Stern (1939) has suggested that there may be specific periods of 

 gene activity. The orderly sequence of developmental processes would 

 then be a direct result of an orderly, timed, genie activity. He also 

 notes that whereas some genes produce phenotypic effects at an early 

 stage in the individual development, others may show a continued 

 activity. To relate the developmental sequence to the hypothetical 

 orderly activity of genes, however, merely restates the problem in other 

 terms, or at most puts it one stage further back : it does not account 

 for the assumption of specific form in a developing organism. 



CHEMICAL ASPECTS OF EMBRYOGENY 



Textbooks of botany and of plant physiology contain little informa- 

 tion on the chemical composition of the zygote, the young embryo, or 

 the surrounding tissues. Nor do they tell us much about the metabolic 

 processes that may be involved in the germination of a spore or the 

 growth and division of a zygote. Equally, they have little to say about 

 the composition and metabohsm of the distal meristem that is estab- 

 lished at an early stage in the embryogeny and persists throughout the 

 vegetative development. These points are emphasised to remind the 

 reader how much remains to be done before any adequate presentation 

 of what is involved in embryogenesis can be made. Admittedly the 

 magnitude and complexity of the task are formidable. But so too are 

 they in the equivalent field of zoological inquiry; yet zoologists have 

 made some progress, even if thus far only the surface of the problem 

 has been scratched. Here the reader is referred to the books of Needham 

 (1931, 1942) and Brachet (1944, 1950), where the results of a very 

 considerable amount of biochemical and physiological work have been 

 recorded. Thus, they deal with the cytochemistry and histochemistry 

 of the mature and fertilised egg and the factors which may affect them, 



