EMBRYOGENESIS IN GYMNOSPERMS 221 



(v) The initial development of small embryos, i.e. those with no 

 free nuclear division, and of cleavage embryos, is essentially, and some- 

 times conspicuously, filamentous, the distal end being the region of 

 sustained protein synthesis and morphogenetic activity. 



(vi) With the exception of the Cycadales, Ginkgoales and Gnetales, 

 cleavage polyembryony is a very general phenomenon. Abundant 

 nutrition no doubt makes polyembryony possible, but the phenomenon 

 should probably be referred to the action of gene-controlled enzymes. 



(vii) In all gymnosperms, large amounts of nutrients are available 

 in the gametophyte and nucellus. The ovum is sometimes of very large 

 size and densely packed with storage materials; but even where the 

 ovum is small, the proembryo and embryo are surrounded by, and can 

 draw on, abundant supplies of nutrients during growth. The nutritional 

 status of the embryonic environment thus seems likely to be of great 

 importance in determining the development of the embryo. The very 

 extensive formation of suspensors is probably an indication not so 

 much by their functional importance as of the quantity and quality of 

 nutrients available during the initial post-fertilisation phase. Only 

 later, when a readjustment in the balance of nutrients may have been 

 effected, does the definitive embryonic development proceed, this 

 involving the formation of a bulky meristematic tissue mass. The 

 delayed formation of the definitive embryo in the gymnosperms is 

 reminiscent of the slow or belated organogenic development in some 

 pteridophyte embryos, 



(viii) Size and form correlations are evident in gymnosperm 

 embryogenesis. Direct correlations have been observed between the 

 size of the seed, the size of the embryo, and the number of cotyledons. 

 The initial presence of an apical cell in some species and its disappear- 

 ance later may also be indicative of a size-structure relationship. 



PHYLOGENETIC ASPECTS 



No close resemblances can be indicated between the embryos of 

 gymnosperms and pteridophytes. Again, while some embryonic 

 developments in gymnosperms are of an angiospermous character, the 

 actual details are not to be matched in angiosperms: they suggest 

 parallel evolution rather than descent from a common ancestor (see 

 Chapter XVI, p. 323). While maintaining a strictly conservative 

 attitude in these difficult problems of phylogeny, an attempt should 

 nevertheless be made to indicate views that are not merely negative 

 and unconstructive. To this end we may note, in the first instance, that 

 gymnosperm embryos share the same general features as all other 

 embryos, i.e. the early determination of polarity, axial development and 

 a conspicuous meristematic and morphogenetic distal pole. On the 



