242 ORR—TRACHEIDES IN STERIPHOMA CLEOMOIDES. 
their walls, in the nucellus of many of the ovules of Steviphoma 
cleomoides. They were particularly obvious in sections stained 
with gentian-violet. 
They occur singly, and in groups of two or three together, 
abutting on the mature embryo sac, and extending from its 
organic base over the whole length of the sac in the direction 
of the micropyle. 
Their form and structure suggest that they are tracheidal 
in nature, although the production of tracheides in sporogenous 
tissue is of such rare occurrence. They are completely isolated 
from the vascular strand traversing the funicle, and there is no 
evidence of any connection with the chalaza. 
As more mature ovaries of Steviphoma were unobtainable at 
the time, the ultimate fate of these elements could not be traced, 
and an examination of the ovules of some of the allied genera 
failed to disclose the presence of similar structures. 
No satisfactory explanation can be found to account for the 
production of these tracheides in the nucellus of Steriphoma, 
and, at present, it is not possible to say whether they play a 
definite part in the economy of the ovule, or are merely survivals 
of a primitive feature traceable to the vascular sporangia of a 
long-extinct ancestor. In either case, their non-occurrence 
in the nucellar tissue of the ovules of allied genera is peculiar, 
although their limitation to one genus of a family is in accordance 
with the findings of other authors. 
EXPLANATION OF PLATE CLXVI. 
Illustrating Mr. M. Y. Orr’s paper on Steriphoma. 
Ovule of Steriphoma cleomoides in longitudinal section. m., micropyle ; 
s., embryo sac ; ¢y., tracheid. 
