EMBRYOGENESIS IN FLOWERING PLANTS 235 



tetrad, and the other four to a hnear tetrad, Fig. 53 (Soueges, 1936, 

 1937). 



To explain the phenomena that underlie the law of destination in 

 terms of the process of growth may be indicated as one of the major 

 tasks in plant embryology. This law is closely related to the preceding 

 one. The determination of the relationships between the initial seg- 

 mentation pattern and the subsequent organogenesis has been described 

 as embryogenergy. 



Embryonal Formulae. Soueges' classical demonstrations of em- 

 bryonic development have led to the general acceptance of his descrip- 

 tive formulae. These indicate the origin and relationships of individual 

 cells during the early embryogeny. The laws of embryonomy indicated 

 above relate to, and have their inception in, these formulae. The 

 formulae facilitate comparisons between embryos, and reveal at once 

 the essential histological similarities and differences. Fig. 55 illustrates 

 the system of lettering as proposed by Soueges. 



EMBRYONOMIC TYPES 



The cellular configuration of the embryo at the four-celled stage, 

 Fig. 55, and the part played by each of these cells in the organogenic 

 development, constitute the basis for a classification of embryonomic 

 types. Schnarf (1929), Johansen (1945, 1950) and Maheshwari (1950) 

 have distinguished five principal types of dicotyledon embryo. 



I, The terminal cell of the two-celled proembryo divides by a longi- 

 tudinal wall: 



(i) The basal cell plays only a minor part or none in the sub- 

 sequent development of the embryo. 



Crucifer {or Onagrad) type. Fig. 55b, c. 



(ii) The basal and terminal cells both contribute to the develop- 

 ment of the embryo. Asterad type. Fig. 55d, e. 



II. The terminal cell of the two-celled proembryo divides by a trans- 

 verse wall: 



{a) The basal cell plays only a minor part or none in the subsequent 

 development of the embryo : 



(i) The basal cell usually forms a suspensor of two or more cells 



Solanad type. Fig. 55f, g. 



(ii) The basal cell undergoes no further division and the sus- 

 pensor, if present, is always derived from the terminal cell. 



Caryophyllad type. Fig. 55k, l. 



{b) The basal and terminal cells both contribute to the development 

 of the embryo. Chenopodiad type. Fig. 55h, j. 



