52 Farmer . — On Isoetes lacnstris , Z. 
in the Gymnosperms, in or Pimts , for example. The 
plerome, in all cases where I have been able to trace it 
clearly, grows certainly from a single initial cell, but from 
the excentric course which the pro-cambial cylinder pursues 
in the root, it is by no means easy to secure sections which 
pass through it in the right plane. The cells which are cut off 
behind the irregularly shaped cell at the apex are readily 
distinguishable on account of their large nuclei and proto- 
plasmic contents, as well as their large size as compared with 
the surrounding cells. The cells thus forming a row, divide 
peripherally and longitudinally, leaving a much larger central 
cell, from which further cells are cut off longitudinally ; this is 
figured by Hofmeister 1 , and also by Naegeli and Leitgeb 2 , 
and in both cases with remarkable accuracy. Hofmeister 
explained the large cell as the apical cell, but a series of trans- 
verse sections shows this view to be untenable (see PL V. Figs. 
5 v 6, 7, 8), and longitudinal sections (Figs. 9, 10, 30) also 
demonstrate clearly the actual state of the case. Hofmeister s 
sections were cut, as a matter of fact, below the actual apex of 
the plerome, and show the early stages of the development of 
the vascular bundle. The photograph (Fig. 30) shows how 
large the central cells are in proportion to those surrounding 
them, and also how they retain the dense protoplasmic con- 
tents and large nuclei which are commonly associated with 
merismatic structures. The xylem-elements of the monarch 
vascular bundle only make their appearance at a relatively 
late period. The two photographs (Pigs. 38, 29) show pretty 
clearly the stages by which this is brought about ; and they 
also show how the phloem-portion of the bundle is well differ- 
entiated before the xylem, which is not as yet formed in either, 
although in Fig. 29 its general future position is distinguishable. 
There is no need to further discuss the structure of the mature 
portion of the root, it having been sufficiently described by 
other investigators, and the split in the cortex has also been 
correctly explained by Bruchmann. As to the origin of the 
lateral roots from the stem, I have as yet been unable to 
1 Loc. cit. Plate XIII. Figs. 3, 4. 2 Loc. cit. Plate XIX. Figs. 3, 6. 
