THE HIGHER CRYPTOGAMIA. 347 



In all the principal features the development of the scale 

 of Isoetes accords with that of the scales of ferns. The 

 first commencement of the multiplication of the single 

 primary cell is essentially the same in both, i. e., it rests 

 upon the alternation of rectangular, longitudinal, and 

 transverse divisions ; besides this, both exhibit the subse- 

 quent intercalary basal cell-multiplication, and the same 

 kind of multiplication of the median cellular layers ; and 

 lastly, in both cases the development of the scales rapidly 

 gets ahead of that of the organ to which they form appen- 

 dages, and they soon die. 



Immediately after the commencement of the formation 

 of the scale, a sheath begins to be formed at the base of the 

 leaf, enclosing the scale and some of the cells underneath 

 it. A horse-shoe-shaped, cushion-like protuberance, open 

 towards the front surface of the leaf, is first formed by the 

 arching outwards of the free outer wall of a girdle of cells 

 surrounding those parts (PI. XLVII, fig. 24). When the 

 intercalary cell-multiplication of the base of the leaf takes 

 place, this protuberance grows up into a tolerably high 

 annular sheath, by repeated division of the apical cell for 

 the time being by means of horizontal septa (PI. XLVII, 



The leaf grows in thickness at the point of origin of the 

 scale ; its fore-side appears bent obliquely inwards (PI. 

 XLVII, figs. 2, 3), close above the base (PL XLVII, figs. 

 2, 3). In its lower part, the vascular bundle which tra- 

 verses it is excentrical, being nearer to the front surface. 



The primary axis of the germ-plant, which at the time of 

 the appearance of the first leaf consisted of only a few 

 cells, increases considerably in length and circumference 

 during the development of the leaf and the formation of 

 its scale. This increase is caused more by expansion than 

 by multiplication of its cells. The axis now r projects 

 considerably from the embryo, in a hemispherical form, 

 and is directed towards the middle point of the pro- 

 thallium ; the leaf is seated upon one of the lateral sur- 

 faces of the embryo (Pl.XLVI,fig. 24; PI. XLVII,figs.l-3). 



A process of cell-multiplication has now commenced 

 upon its opposite lateral surface also : this is the beginning 



