60 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
ovules and possesses also in large numbers special excretory cells 
which become loaded with raphides. | 
Similar cells are found in some @alieae in the immediate vicinity 
of the embryo-sac for some time before and after fertilization, and 
the róle which the contents play in the physiological economy of 
the ovule is by no means clear. 
It may turn out that in both instances the calcium oxalate is 
of some positive value, and the term “excretory ” as used above 
must be regarded as provisional. 
3. In Houstonia each locule of the ovary is provided with a 
club-shaped placenta bearing each a number of ovules. The to- 
pographic relations of a placenta in Houstonia and an ovule in the 
Galieae are the same, and no differences have been observed in 
their origin. 
The ovules in Houstonia are relatively very small and of a very 
simple type in that an zztegument is wholly absent. There is ac- 
cordingly no micropyle and the archesporium becomes deeply : 
buried in the nucellar tissue by the growth of the capping cells and 
of the tissue at the sides of the archesporium toward the apex of 
primordium. The archesporium further consists of but one func- 
tional embryo-sac-mother-cell. 
Two questions arise concerning the above comparison ; first, 
whether we may regard the ovule of the Galieae and the placenta 
with its ovules in Houstonia as morphological equivalents, and 
secondly, whether we have here a case of correlation. 
To the first we can give no positive answer at present since a 
more careful study of the origin of the primordia is needed. If 
such should prove to be the case we should have a sort of branch- 
ing or compound sporangium, a most interesting condition if true. 
: To the second question a more decided answer in the affirma- 
tive may be given in view of the fact that where the placenta is 
formed the ovules are arrested both as regards the integument and 
archesporium and in total relative size. That the archesporium is 
really suppressed appears from the fact that commonly about seven 
embryo-sac-mother-cells are laid down, all but one of which fail to 
reach the normal size. 
It must be added that the belief that a simple naked ovule, 
that is, one without any integument, is correlated with the para- 
Li 
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