390 



A. B. STOUT 



Such conditions emphasize the marked individuaHty 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 t\vo 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 (191 5) 

 have especially emphasized, an interaction between the haploid 

 pollen-tube and the diploid tissues of the pistil. There may be somt 

 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 



