390 
A. B. STOUT 
Such conditions emphasize the marked individuaUty of the develop- 
ment of conditions giving incompatibihty. The conditions are 
fundamentally physiological and arise apparently in connection with 
the differentiation of the two sets of so-called sex organs. Important 
to an understanding of the facts of differentiation here involved are 
the phenomena of cross-incompatibilities. Three sister sporophytes 
which are quite identical in all vegetative characters may possess sex 
organs that are incompatible to the extent that complete self-sterility 
is in evidence; no. i may be incompatible with the male sex organs 
(microgametophytes and gametes) of no. 2, but compatible with those 
of no. 3. This difference in relation is certainly indicative of dif- 
ferences in the physiological qualities of the two lots of male gameto- 
phytes. Conversely the microgametophytes and gametes produced 
by a single sporophyte may act quite differently on the female sex 
organs borne on two other sporophytes, being compatible in one case 
and incompatible in the other. This indicates, likewise, a difference 
in the condition of the two sets of female organs (including pistils). 
Furthermore, the data as to the occurrence of cross-incompatibilities 
in chicory even indicate that reciprocal crosses between two plants 
may give quite the opposite results, showing that the relations of the 
two sets of sex organs may not be interchangeable. 
In such phenomena we may recognize a loss of sex-vigor which is 
concerned with the function of gametophytes and gametes. The 
decrease in fertility is entirely independent of a decrease in the pro- 
duction of spores. Furthermore, there appears to be full and complete 
development of the macrogametophyte and its egg; its development is 
certainly not inhibited by the condition of the pistil in which it develops. 
There is no evidence that the microgametophyte is not fully developed 
with reference to its differentiation. Although often involving a 
decreased vegetative growth of the pollen-tube, the inhibition appears 
fundamentally to involve function. 
The reactions involved in self- and cross-incompatibilities do seem 
to involve, to some extent at least, as Jost (1907) and East (J915) 
have especially emphasized, an interaction between the haploid 
pollen-tube and the diploid tissues of the pistil. There may be some 
question as to what extent these relations are involved. 
Incompatibilities are evidently indicated not only by an inability 
to produce embryos, but also sometimes by a feeble viability of those 
that are produced. This death of embryos among seed produced by 
