302 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the cotyledons and those in the center to the stem tip, offering some 

 resemblance in this respect to Geum (Fig. 149) and Myosotis (Fig. 

 155), which have already been described. 



Attention may finally be called to the fact that although Soueges 

 lays much emphasis both on the constancy in the destiny of the 

 cells of the four-celled proembryo and on their significance in indi- 

 cating relationships between different families and genera, his con- 

 clusions have not been supported by other workers. For example, 

 Borthwick (1931) points out that in Daucus each cell of the four- 

 celled proembryo does not always give rise to the same part of the 

 mature embryo. Bhaduri (1936) also states that "the method of 

 origin of the different parts of the mature embryo varies in different 

 and sometimes even in the same species of Solanaceae." 



UNORGANIZED AND REDUCED EMBRYOS 



Whatever their mode of origin and development, as a rule all 

 embryos become differentiated at maturity into three main parts, 

 viz., root tip, stem tip, and cotyledons (or cotyledon). In some 

 plants, however, the embryo remains small and rudimentary even 

 until the shedding stage of the seed. This condition is especially 

 prevalent in the families Balanophoraceae, Rafflesiaceae, Gentia- 

 naceae, Pirolaceae, Orobanchaceae, Burmanniaceae, and Orchida- 

 ceae, and appears to be associated in some degree with a parasitic or 

 saprophytic mode of life. 



In the Orchidaceae the embryo is almost always a simple or ovoid 

 mass of cells in which even the differentiation of the primary layers 

 takes place only after the seeds have been shed. Swamy (1949) 

 recognizes two fundamental types of development. In the first 

 type (Fig. 174A-0), which is the more frequent, the zygote divides 

 by a transverse wall to form two cells, of which the basal again 

 divides transversely so as to result in a proembryo of three cells. 

 The upper cell adjacent to the wall of the embryo sac frequently 

 gives rise to one or more suspensor haustoria, which become very 

 prominent and aggressive structures in some species. The terminal 

 cell divides vertically to form two daughter cells, which by further 

 divisions give rise to the greater part of the embryo, the rest being 

 contributed by the middle cell. In the second type of development 

 (Fig. 174P-C7), which has been reported only in a few genera like 

 Cymbidium, Eulophia, Geodorum, and Stanhopca, the zygote may 



