30 Lang.—Studies in the Morphology and 
nature of the stele in the hypocotyl. On the separation of the leaf-trace 
xylem (PL III, Photo 42), the outer xylem becomes complete around the 
inner xylem of the stele, which again forms a solid cylinder. For some 
distance the stele, now belonging to what may be termed the first internode 
of the epicotyl, maintains this appearance. Then parenchyma again appears 
in the central xylem, replacing many of its elements, and shortly after this 
the second leaf-trace becomes clearly recognizable, forming the dorsal half 
of the stele (PL III, Photo 43). In this section the xylem of the leaf-trace 
is again seen to be endarch, and to be continuous with the outer xylem 
of the stele. This outer xylem is one or two tracheides thick in the lower 
portion of the stele, and surrounds the small ‘ mixed pith *, consisting of 
a number of parenchymatous cells, and some three or four tracheides of the 
inner xylem (x.i.). The separation of the leaf-trace xylem would evidently 
leave a gap in the outer xylem, but before this takes place (PL III, Photo 44) 
the inner xylem is found to be much more strongly developed, rendering 
the stem portion of the stele almost solid again. Indeed, before the leaf- 
trace xylem completely separates (PL III, Photo 45), the outer xylem in the 
stem stele becomes continuous, so that on the departure of the trace, the 
stele is left with a complete solid xylem strand consisting of inner and 
outer xylem. In the preparations for the departure of the third leaf-trace 
medullation again takes place by replacement of the inner xylem by 
parenchyma. Thus in PL III, Photo 46, the xylem of the nascent third 
trace is recognizable, forming the dorsal portion of a stele that has a small 
pith, scattered tracheides of inner xylem round this, and a continuous tube 
of outer xylem. 
It is unnecessary to follow the changes node by node, for the facts just 
given will be sufficient to show the rhythmical nature of the changes in the 
stele associated with the leaf-trace departures. It should be mentioned, 
however, that this stele never became quite solid above the departure of the 
third trace, a small pith persisting. The second root arose from the ventral 
surface of the rhizome, just after the first leaf-trace separated from the stele, 
and the third root was inserted shortly after the stage reached in Pl. III, 
Photo 46. The leaf-traces of this plant were endarch, and showed no 
adaxial completion of the xylem, but cases in which this was effected have 
been observed in other young plants. 
The progression from the juvenile to the adult type of rhizome. The 
comparison of the stelar structure of young plants and branches on the one 
hand, and of rhizomes of different diameters, but with the usual adult type 
of stele, on the other, reveals an essential correspondence. The chief 
differences in the adult type, apart from size, concern the definite and 
persistent medullation ; the development of the elements of the inner xylem 
as pitted tracheides and the occurrence of an internal endodermis. 
The origin of the pith in the seedlings and other small rhizomes 
