in the Genus Primula. 
321 
important influence of the leaf-traces on the phenomena of 
the transition, and, indeed, on the vascular system throughout 
the whole plant. On this account he regards the polystely, 
when present, as having originated by the continued bifur- 
cation of the central cylinder found in the lower part of the 
stem. He speaks of it as flattening itself out and constricting 
itself in the middle until it becomes nipped into two 1 . I have 
not seen anything that would lead to a similar conclusion. 
The transitional phenomena in the seedling, and also the 
extreme variability in certain species, of the most important 
characteristics of polystely, give strong support to the opinion 
that polystely is not a primitive feature of the group of 
Primulas in which it is found, but a comparatively recent 
modification. Monostely is undoubtedly to be regarded as 
the fundamental type, and Van Tieghem appears to be correct 
in laying so much stress upon the incapacity of the narrow 
epicotyledonary stele, in the polystelic section of the genus, to 
expand itself sufficiently to satisfy the needs of the plant, an 
inability which he regards as the ultimate cause of their 
polystely (1. c., p. 304). 
Since the epicotyledonary stele of the monostelic section of 
the Primulas has retained its ability to expand with the 
growth of the plant, that of the polystelic section must have 
lost this same power, which at one time it also possessed, and 
it must have done so at a period anterior to the first appear- 
ance of polystely in the section. The only way in which this 
could have come about is by the prototypes of the polystelic 
section having been at one time subjected to very different 
conditions from those of the monostelic section, those con- 
ditions being such as were unfavourable to the production of 
much vascular tissue, and therefore favourable to the estab- 
lishment of a very narrow central cylinder throughout the 
plant. Now a recurrence to those conditions, in which an 
increased amount of vascular tissue would be advantageous, 
could be met, as regards the leaf-traces, by an increase in 
their size or in their number. But if such an increase actually 
1 Ann. des Sc. Nat. Bot., 7 ser., T. Ill, pp. 282 and 295. 
