56 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
Houstonia 
(PLATE 14) 
The two species of the genus Houstonia which have been 
studied are Houstonia coerulea and H. longifolia, both of which 
have been found to be in essential agreement. 
The style, which in Houstonia is single, is provided with two 
stigmas, and arises by the concrescence of two ridge-like fun- 
daments which develop, as in the GaZeae, laterally and oppo- 
site to each other, in the inside of the hollow torus (s, figs. 7, 2 
and 3). During concrescence a transverse ridge (figs. 1, 2 and 3) 
is formed on the inside of the ovary roof, which, by downward 
growth, ultimately divides the originally single ovarial chamber 
into two locules. This ridge, which may be described as hanging 
from the roof of the ovary, unites along its free lower border 
with a parallel, low broader ridge developed in the floor of the 
ovary. This floor ridge (b, figs. 2 and 3), which must be regarded 
as representing two fused carpellary margins, is not evident until 
the two rounded fundaments of the placentae (5, fig. r) have 
reached considerable size. These two structures have the same 
position and appearance as the fundaments of the ovules in the 
Gaticae. In the latter forms, however, the two carpellary margins 
produce each a single ovule ; in Houstonia a knob-shaped placenta 
is formed in each locule occupying the same topographic relations 
as the ovules in the Ga/reae. 
Each placenta soon begins to show a number of protuberant 
growths, which may early be recognized as the primordia of 
ovules. The development of these is of peculiar interest. 
Up till the time when the archesporium is to be distinguished 
the course of development of the ovule is like that in the ۰ 
A single row of epidermal cells cap the archesporium, but these 
cells, which in the GaZeae remain without further division, in 
Houstonia suffer periclinal divisions (Ag. 4), as do also the 
adjacent epidermal cells. This process is continued until the 
archesporium comes to lie in the middle of the nucellar mass. Vo 
integument at all is developed ; indeed, the regular periclinal cell 
divisions which characterize the early stage in the development of 
the integument in the Ga/zeae are absent. The suppression of the 
integument accounts for the absence of the micropyle. 
