Apical Meristems of the Roots of Monocotyledons . 121 
section, however, is shewn in Fig. 4, taken at the stage when 
the development of the vascular elements has begun. The 
pericycle is seen as a distinct row enclosing five groups of 
fairly large cells about to become xylem elements, and five 
alternating groups of smaller cells, the initials of the phloem. In 
the centre is a large axial cell which will give rise to vascular 
elements. 
Van Tieghem (2) (Plate XXXIII., Figs. 510 to 514) figures 
longitudinal sections of young lateral roots of Alisma. The chief 
points of difference between his figures and my own are that while 
the “ digestive sac ” is clearly indicated in Van Tieghem’s drawing 
my preparations do not shew it. From Van Tieghem’s figures one 
would imagine that that the root-cap (“epidermis”) and the 
“ digestive sac ” had the same origin. Moreover Van Tieghem 
evidently finds two initial cells in the plerome cylinder, whereas I 
could find only one in my preparations. 
Butomus umbellatus. Figs. 5 and 6. 
Fig. 5 represents a root at a fairly young stage. The plerome 
shews very clearly one large initial cell from which the remainder 
of the plerome is cut off, and distinct signs of a central axial row. 
Another large initial cell gives rise to the periblem and the derma- 
togen. The calyptrogen has divided at the apex to form a two¬ 
layered root-cap. 
An older stage is seen at Fig. 6. The central axial row of the 
plerome is very distinct, being formed of large isodiametric cells 
which cut off no lateral segments. The pericycle in this case is 
evidently formed at the first division of the apical cell ; the endo- 
dermis is not yet formed, and the root-cap is six layers in thickness. 
Transverse sections, of which none are figured, shewed the 
the same general characters as Alisma. 
In Van Tieghem’s (2) figures of the development of the root in 
Butomus (Plate XXXIV., Figs. 525 to 530), the same critcisms are 
applicable as have already been made with reference to Alisma. 
Vallisneria. Figs. 7 to 9. 
A young stage of a root in longitudinal section is shewn at 
Fig. 7. The plerome consists of a central row surrounded by a 
single layer of cells and arises from a single initial cell. The 
periblem and dermatogen also arise from a single initial cell, from 
which is cut off on either side a segment which remains undivided, 
the second lateral segment dividing parallel with the root surface to 
