EMBRYOGENESIS IN GYMNOSPERMS 177 



lower groups. In these observations, something of the biological 

 significance of the seed habit, especially in its nutritional aspects, begins 

 to be apparent: the ovum, and the germ formed from it, have a nutri- 

 tional endowment that is quite unequalled in any pteridophyte, where 

 the young embryo is dependent on the slender resources of an insignifi- 

 cant prothallus. This, of course, is a general observation; it is intended 

 to emphasise a particular aspect but it requires some qualification. As 

 we shall see in later chapters, there are many angiosperms in which the 

 seed is very small and its contained germ ill-developed. 



On the completion of the mitosis which gives rise to the egg nucleus 

 and the ventral canal nucleus, and with the dissolution of the latter, 

 the ovum enters on its final phase of maturation. The nucleus now 

 moves towards the centre of the ellipsoidal egg, and becomes very large, 

 up to 500/^ in the cycads (Chamberlain, 1935). The cytoplasm becomes 

 very dense, most of the vacuoles disappear, leaving what has been 

 described as a kind of fibrillar appearance, and densely packed storage 

 materials, including starch, oil and proteins, can be identified. This is 

 the condition of the ovum as it awaits fertilisation. 



In Finns, also, the ovum is of considerable size and is surrounded 

 by a nutritive archegonial jacket; but in some other Coniferales the 

 ovum may be quite small, as in Sequoia {see Chapter XII). 



The shapes and relative positions of the prothallus and archegonia 

 are such as to indicate that physiological gradients not only operate 

 between the periphery and the centre of the ellipsoidal ovum but also 

 along its main axis, these making for important differences in reaction 

 at the outer, or micropylar end, and the inner, or basal, end. Such 

 polarity may indeed have been established before, or during, the 

 formation of the linear tetrad of megaspores. In all gymnosperms the 

 embryogeny is typically endoscopic: the embryonic cells which give 

 rise to the shoot apex lie adjacent to the basal end of the archegonium, 

 the suspensor being formed towards the micropylar end. The young 

 embryo grows downwards into the prothallus (or 'endosperm') during 

 its Subsequent development. 



Terminology. For convenience, the region of the zygote adjacent 

 to the neck of the archegonium will be referred to as the upper region, 

 or upper end, and the opposite end as the basal region or end. The 

 embryo apex is always formed at the basal end and is endoscopic, i.e. 

 develops away from the archegonium neck. 



THE CYCADALES 



The order Cycadales comprises some 85 living species, these being 

 the contemporary representatives of a conspicuous and important 

 group of plants that flourished in the Mesozoic Period. The genera 



