i26 Daisy G. Scott. 
the cells in the next outermost ring alternate with those of the next 
projecting between those of the other layer as is shewn in Zostcra 
(Fig. 13). 
As it grows older the cells of the middle row become wide, 
thick-walled, and elongated, exhibiting the spiral markings 
characteristic of protoxylem, Other cells of a similar appearance, 
but of smaller size, are to be seen in the surrounding tissue in 
Limnocharis (Figs. 17, 19). Here the first cells to become lignified 
are those next the pericycle, and this lignification soon follows in 
the cells immediately inside. Small groups of phloem elements 
appear between the protoxylem groups. It may be noted that 
the xylem elements are developed centrifugally as Buscolione (5) 
and Pirotta (6) have already stated. It is true that the lignification 
may take place centripetally as is shewn in Fig. 19, but the actual 
formation of the elements takes place from the centre to the 
outside. Many of the figures in longitudinal section shew the 
central row of cells gradually elongating to form an axial vascular 
row. Fig 18 in particular shews that the lateral vascular elements 
being formed at a later stage. 
IV.— The Periblem and the Dermatogen. 
The common initial for the periblem and the dermatogen is 
usually situated just above that for the plerome. It may cut off a 
segment on either side which at first remains undivided, or this 
segment may at once divide tangentially, the interior cell seen in 
longitudinal section giving rise to a single external layer of cells, 
the dermatogen of which remains undivided, the posterior cells all 
dividing to complete the periblemic tissue. However three rows 
may be formed at once, as in Butomus (Fig. 6). 
The dermatogen cells usually remain the same size, and are 
quadrangular (in longitudinal section), throughout the whole length 
of the root. 
The cell giving rise to the periblemic tissue may divide 
transversely once or twice and become broader as it gets more 
room, as in Limnocharis (Fig. 17), or it may divide at once by a 
periclinal wall on the side away from the plerome. This first 
external segment of the cortex forms, usually by transverse 
divisions, a single layer of cells ( Limnocharis , Fig. 17), the exodermis, 
or outside layer of the cortex. This layer is peculiar in that its 
cells widen very much as they recede from the apex, and at length 
may divide equally as in Limnocharis (Fig. 18). The cell from 
which this segment is cut proceeds on the same plan, and first 
