EMBRYOGENESIS IN GYMNOSPERMS 193 



appear. In the presence and persistence of an apical cell, morphologists 

 have claimed support for the idea of a pteridophyte ancestry of the 

 Coniferales. 



The growing embryonic tissue both withdraws materials from the 

 prothallus and secretes enzymes into it. As a result the prothallial 

 tissue becomes disintegrated and liquefied, a zone of plasmatic material 

 typically surrounding the embryo apex. Also, starch disappears from 

 the prothallial cells before the embryo encroaches on them. As the 

 prothallial tissue offers some resistance to the passage of the embryos, 

 the suspensors come apart and become closely coiled and compressed 

 and fill the disintegration cavity round the embryo. After the embryo 

 has attained its maximum length, the suspensor and the remains of the 

 egg and nucellus form a dry cap which may protect the root and basal 

 end of the embryo when it breaks through the testa at germination 

 (Chamberlain, 1935). 



It may be inferred that while the embryo proper still consists of 

 undifferentiated meristematic cells, the biochemical pattern that 

 underlies and precedes the visible structural development is, never- 

 theless, beginning to be determined. Fig. 37. At any rate, soon after 

 this stage, the distal region of the embryonic mass begins to differentiate 

 into the shoot apex and cotyledons, while the proximal region becomes 

 organised as a root. In the inception of the primary root, a small group 

 of root initials can first be distinguished, and as these grow and divide 

 the tissues of the periblem, root-cap and plerome become differentiated. 

 Fig. 37l. a dermatogen is not recognisable in the early embryogeny. 

 In Pinus many cotyledons are formed. Some investigators, e.g. Hill and 

 de Fraine (1906-1910), have suggested that this is a derivative condition 

 from a dicotyledonous ancestor, but others have taken the opposite 

 view (Dorety, 1917; Chamberlain, 1935). Recent comprehensive 

 investigations by Buchholz (1918-1933) indicate that polycotyledony 

 is the primitive condition, the smaller cotyledon numbers being due to 

 fusion. The several cotyledons in Pinus appear as primordia round the 

 shoot apex before any vascular strands associated with them can be 

 observed, Fig. 42 (Chamberlain, 1935). 



In Pinus, the mean number of cotyledons is eight. P. banksiana (a 

 species with an apical cell in the mature embryo) has three to six, and 

 P. contorta from two to eight. On the other hand P. lambertiana has 

 from twelve to eighteen, and P. sabiniana from seven to eighteen. It 

 would be interesting to know, from a morphogenetic study of these and 

 other species, if the small cotyledon number is associated with a small 

 apex and the large number with a large apex, i.e. indicating a size and 

 form correlation. Such information would be important in relation to 

 Turing's (1952) diffusion reaction theory of morphogenesis {see also 



