EMBRYOGENESIS IN FLOWERING PLANTS 277 



the endosperm. On further growth, a vascular strand is differentiated 

 in the cotyledon and becomes conjoined with the vascular strand 

 differentiated in the first root. Now, this kind of morphological system, 

 comprising a vasculated cotyledon (or prophyll), a root in vascular 

 continuity with it, and a poorly organised shoot apex, is what is found 

 in various eusporangiate ferns, e.g. species of Marattiaceae and Ophio- 

 glossaceae. And, as in these ferns, the shoot apex in grasses, however 

 small and inconspicuous it may be initially, is nevertheless the centre of 

 all the ensuing axial development {see Chapter IX). Furthermore, we 

 may recall that in ferns, lycopods and Selaginella, various parts of the 

 embryo may become greatly swollen and modified in relation to 

 supplies of nutrients. Similarly, in grasses, with their very well developed 

 endosperm, it is not surprising that some parts of the embryo become 

 more or less extensively modified. From Merry's illustrations of the 

 young embryo of Hordeum, in which the scutellum is very short and 

 the nascent shoot apex in a relatively distal-lateral position, it is not 

 difficult to suppose that, at an earlier stage, both the incipient scutellum 

 and shoot apex had occupied distal positions. But, in relation to the 

 underlying biochemical pattern, the apical site grows slowly, the foliar 

 site rapidly. 



In the grasses, as in other monocotyledons, the cotyledon appears 

 to be truly terminal: there is an even gradation in cell size from the 

 large-celled suspensor to the small-celled distal region. Nevertheless, 

 this club-shaped embryo has in it a characteristic biochemical asym- 

 metry, the shoot apex being always on the side away from the endo- 

 sperm. If we assume, as we may well do, that the metabolic pattern 

 which underlies the inception of the shoot apex and the first leaf is 

 determined while the embryo is still very small, the difficulties in accept- 

 ing the apex as the terminal pole of the embryonic axis largely disappear. 

 The cotyledon (scutellum), the coleoptile, and the first foliage leaf will 

 all stand in the normal relation of lateral organs to this axis, though 

 their form and structure may be greatly modified by the nutritional and 

 other factors which are at work during their respective developments. 

 According to Avery: 'The fact that the embryonic stem tip appears 

 to arise from the lateral face of the immature cotyledon need not be 

 construed as meaning that the cotyledon is terminal. A more logical 

 explanation, in view of the position of the suspensor in the mature 

 embryo, would be to consider the origin of the growing point of the 

 stem as having been delayed in development until after the cotyledon 

 has attained considerable size.' Elsewhere, in Lycopodium and in 

 eusporangiate ferns, examples of delay in the histological organisation 

 of the shoot apex have been noted, and in them also the position of the 

 axis has been temporarily obscured. 



