EMBRYOGENESIS IN FLOWERING PLANTS 233 



development of an organism is essentially due to gene-determined 

 physiological factors and to various physical and environmental factors. 

 The relevant fundamental laws would relate to the action and interaction 

 of these factors and would attempt to relate the energy involved to the 

 organisation developed. But the laws as stated do contribute to a 

 description of, and afford a formula for, the orderly embryonic develop- 

 ment. 



Johansen considers that the law of parisimony may be the funda- 

 mental law and that all the others are derived from it. In this con- 

 nection we may note that Haberlandt (1914) laid emphasis on what he 

 described as the principle of economy of material and the principle of 

 efficiency as pervading all development in plants, the latter principle 

 being tantamount to a natural law. As stated by Johansen, the law of 

 parsimony might be felt to carry some teleological implication. The 

 law of origin calls attention to the fact that the embryogeny of any 

 species is usually characterised by great regularity and orderliness both 

 in the overall growth and in the sequence of cell division. This is, of 

 course, a very striking fact and one which should not just be taken for 

 granted. It is indicative of the very precise control of development by 

 genetical and other factors. The law of numbers takes cognisance of 

 the fact that each species has its own specific pattern of differential 

 growth and that this becomes apparent in the characteristic histological 

 pattern of the embryo. Johansen states that : 'The number of (cellular) 

 elements engendered conditions the rapidity of the segmentation ; it is 

 the essential factor of determination.' The writer would rather say that 

 gene-controlled metabolism and differential growth, together with the 

 various factors which become incident in the enlarging embryo, 

 determine the number and size of cells produced in different regions of 

 the embryo. 



The law of disposition, as stated by Johansen, appears to have 

 teleological implications. With rare exceptions, the first division of 

 the elongating zygote is by a transverse wall. The next division in the 

 terminal cell may be either transverse or longitudinal. In many embryo- 

 logical works the segmentation of the enlarging embryo is treated 

 purely as a descriptive morphological study. This is an indispensable 

 initial stage : we must know what the growing embryo is like morpho- 

 logically and histologically before we can begin to explore the under- 

 lying processes. The whole study will gain in vitality and value in 

 so far as the visible anatomical developments are regarded as the result 

 of processes of growth, beginning with a zygote of specific constitution. 

 This law, as also the law of destination, once again reminds us of the 

 extraordinary precision and fidelity with which the histological pattern 

 is repeated during the individual embryonic development. There are 



