100 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
3. Richardsonia pilosa—The conductive tissue as in Diodia 
teres, but somewhat different histological relations exist in the fusion 
tissue of the ovary septum. The mechanical relations are such 
that the strophiolar conductive tissue above the funicle is com- ` 
pressed between the ovary wall and the ovule. Below, 2. e., toward 
the micropyle, the pressure is reduced. The pollen tube enters 
the conductive tissue at the base of the stylar portion of the parti- 
tion, travels by the intercellular method in the same till it reaches 
the micropylar region where it often though not always bends to- 
wards the surface. 
The above-named facts speak for the mechanical conditions in 
the ovary as important factors in determining the course of the 
pollen tube, whether intercellular or not. 
In review the following results and conclusions ate presented : 
I. In Richardsonia pilosa and Diodia teres the course of the 
pollen-tube is chiefly intercellular, a phenomenon clearly secon- 
dary in view of the fact that in a closely related species, Diodia 
Virginiana, the pollen tube pursues its path freely in the “ovary 
cavity in that part of its course. 
An important factor in determining such behavior of the pollen 
tube is found in the mechanical relations (pressure) existing be- 
tween the ovule and the ovary wall. This is added as evidence 
supporting the contention of Murbeck to the effect that the inter- 
cellular mode of growth of the pollen tube is a matter of physio- 
logical meaning. 
Inasmuch as such a difference in the behavior of the pollen- 
tube as has been pointed out may occur between two closely re- 
lated species of the same genus, I infer, contrary to the view of 
Nawaschin, that the behavior of the pollen tube is of very limited 
significance—indeed of no practical significance—from a phylo- 
genetic viewpoint. The condition is analogous to that of the 
archesporium in the angiosperms, which occurs, so far as at 
present known, in eight genera of the Rosaceae and in a dozen 
others widely scattered through the whole range of forms from 
the Rubiaceae (5, 6) to Casuarina.* 
This statement does not preclude the possibility that Nawas- 
> AON ee 
A A summary of the facts and literature pertaining thereto will be found in paper 
y Murbeck and Koernicke already quoted, 
