io A. G. Tansley. 
is retained throughout the stem, the stele merely increasing in size 
in relation to the larger and larger leaf-traces that have to be 
supplied. 
Of the ontogenetic history of the Lindsay a -type of stele we 
know, unfortunately, nothing ; but when we pass to the solenostelic 
type we find that the first formed stele is again a protostele, giving 
off simple leaf-traces, that it then passes through a Lindsaya-pha.se, 
in which an internal phloem, continuous with the inner phloem of 
the leaf-trace appears in the centre of the stele. This internal 
phloem may, in the first instance, form a series of 4f pockets ” 
ending blindly below, one in connexion with each trace (Fig. 98). 
Fig. 98. Alsophila excelsa. Diagrammatic median longitudinal section 
through stele of young plant showing breaking up of protostele by phloem and 
ground-tissue pockets. After Gwynne-Vaughan. 
These pockets, at first independent, soon join as we pass up the stem, 
to form the continuous core of phloem characteristic of the Lindsaya- 
type. Then an internal endodermis, continuous with the outer 
endodermis at the “ axil ” of the trace and enclosing a pocket of 
ground-tissue continuous with the cortex, appears, extending 
downwards into the stele from in front of the point of insertion of 
each trace. A continuous pith is eventually formed by the 
coalescence of these pockets, and the typical solenostelic condition 
is thus arrived at. 
In the dictyostelic forms the early history is the same; the 
