XIV ISOETACE^ 549 



first clearly recognisable, show that the foot is not clearly de- 

 fined, as the basal wall early becomes indistinguishable from the 

 displacement due to rapid cell division in the axis of the embryo. 

 It projects but little, and the cells are not noticeably larger than 

 those of the cotyledon and root. 



As the cotyledon lengthens it becomes somewhat flattened, 

 and in the later stages its increase in length is due entirely to 

 basal growth. Even in very young embryos a distinct epi- 

 dermis is evident in the leaf, and about the time that the ligule 

 is formed the first trace of the vascular tissue appears. This 

 consists of a bundle of narrow procambium cells, which lie so 

 near the centre of the embryo that it is impossible to assign it 



F. 



Fig. 317.— Median longitudinal section of an embryo~of the same species shortly before 

 the cotyledon breaks through the prothallium; lettering as in the preceding, X300. 



certainly to either root or leaf; indeed it sometimes seems to 

 belong to one quadrant, sometimes to the other. From it the 

 development of the axial bundles of cotyledon and root pro- 

 ceeds, and by it they are directly united. The section of the 

 central cylinder of the leaf is somewhat elliptical, and it does not 

 extend entirely to the end. Its limits are clearly defined from 

 the periblem, in which the divisions are mainly transverse and 

 the cells arranged in regular rows. 



The primary xylem consists of small spiral and annular 

 tracheids at the base of the leaf, and from these the formation 

 of similar ones proceeds tow^ards the tip. Their number is 

 small, even in the full-grown leaf, and they are the only differ- 



