33 
Anatomy of the Ophioglossaceae . Ill . 
relative size of the rhizome and stele after the departure of the third leaf- 
trace is seen in Text-fig. 8 , D. The rapidity of the transition in this rhizome 
can be associated with the greater size of the stele even at the base, and 
presumably with more effective conditions of nutrition. The comparative 
sizes of stele and rhizome from the base to the attainment of the full diameter 
can be followed in the diagrams in Text-fig. 8. When the full size was 
attained (Text-fig. 8 , e) the stele showed the size and complexity of the 
fully adult type, but had no internal endodermis. 
Condensation of stelar structure from the adult to the juvenile type . It 
is usual to find the primary structure of a young plant or a branch con¬ 
tinuing on the up-grade of elaboration until the adult type is reached and 
then maintained. It was of interest, as bearing on the significance to be 
Text-fig. 8. Selected transverse sections to show the progression in size and stelar complexity 
in a rapidly widening piece of rhizome. A, B, showing departure of the first leaf-trace; C, at the 
level of the first vestigial bud and showing the second leaf-trace on the left; D, the leaf-trace at 
the clepsydroid stage; E, the full-sized rhizome showing a dividing leaf-trace at the periphery 
of the cortex. I. 1 , I. 2 , /. s , leaf-traces of the first three leaves ; r. 1 , stele of the first root; l.t., dividing 
leaf-trace of full-sized region; px., protoxylem group of next leaf-trace. 
attached to this progressive elaboration of the stele in the ontogeny, to find 
a converse change exhibited by certain rhizomes, in which a condensation 
or reduction from the adult to the juvenile type of stele could be traced. 
This was associated with diminution in size of the rhizome as a whole, and 
may reasonably be regarded as due to growth under less favourable 
conditions of nutrition. 
The amount and nature of the condensation will be evident on com¬ 
paring transverse sections of the stele of the same rhizome at different 
levels, photographed to the same scale. Thus Photos 51 and 52 on PI. Ill 
are respectively further from and nearer to the apex of the same piece of 
rhizome. The former shows the usual structure of a small rhizome of adult 
type, with a definite pith surrounded by a xylem-tube with well-developed 
outer and irregular inner xylem. This rhizome actually had an imperfect 
internal endodermis. Passing towards the apex of the piece of rhizome 
D 
