LYCOPSIDA 8l 



growth proceeds, the orientation of the embryo changes so 

 that the first leaf and the stem apex are directed upwards, 

 while the first root is directed obliquely downwards. It is of 

 interest that there is no quadrant specifically destined to 

 produce a stem apex, and that it appears relatively late in a 

 position somewhere between the first leaf and the first root. 

 In some species, there are no clearly defined quadrants at all. 



Despite the absence of a suspensor, the embryology of 

 Isoetes may be described as endoscopic, since it is from the 

 inner half of the dividing zygote that the shoot is ultimately 

 formed. 



For some time, the young embryo continues to be en- 

 closed within a sheath of prothaUial tissue which grows out 

 round it, but ultimately the various organs break through 

 and the first root penetrates the soil. 



A chromosome count of n= lo has been obtained in one 

 species of Isoetes, and of n = 54-56 in several others. 



Isoetes is clearly a remarkable genus, not only in its 

 pecuhar method of secondary thickening, but also in the 

 fact that all its leaves are, at least potentially, sporophylls. 

 For this reason, some morphologists regard the upper half 

 of the corm as representing a cone axis. The lower half they 

 regard as a highly reduced rhizomorph, homologous with 

 Stigmarian axes, and this is supported, not only by the 

 regular arrangement of the roots on the corm, but also by 

 the extraordinary similarity of the roots to Stigmarian root- 

 lets internally. If this view is correct, then the stem, as such, 

 must have become completely suppressed, along with its 

 leaves. 



Stylites was unknown until 1954, when it was first dis- 

 covered, forming large tussocks round the margins of a lake 

 at an altitude of 4,750 m in the Peruvian Andes. Since then, 

 it has been examined in great detail by Rauh and Falk^^, 

 who claim that there are two species. Stylites is no less re- 

 markable than Isoetes, for it likewise exhibits a kind of 

 anomalous secondary thickening, though less active, and all 



