552 



The Journal of Heredity 



in fact, any environmental factor in 

 the broadest sense of the term. Jeffrey 

 recently has pointed out the widespread 

 occurence of morphologically imperfect 

 pollen in many plants, due, as he says, 

 to the hybrid nature of such plants. 

 Whether the same reasoning can be 

 applied to account for morphologically 

 defective pollen in all fruit varieties is 

 still a question; apparently it accounts 

 for some instances. IVIorphological im- 

 perfection is also of no uncommon occur- 

 rence among ovules, as is readily observ- 

 able on inspecting the ovaries of most 

 dotible flowers or of many varieties of 

 fruits. There arc degrees of imperfec- 

 tion, ranging from merely a lack of 

 proper development of the egg apparatus 

 to an almost complete degeneration of 

 the entire ovule. According to Coit, 

 some normal embryo sacs are produced 

 m occasional fruits of both Washington 

 Navel and Satsuma oranges. If dis- 

 integration of the embryo-sacs takes 

 place, it may occur before the formation 

 of the mcgaspores, but usually not until 

 afterwards. He also states that viable 

 pollen of parthenocarpic varieties is 

 either entirely wanting or is very 

 scarce. 



vStrictly dioecious forms unless par- 

 thcnogcnetic or apogamous are neces- 

 sarily self -sterile. In the very interest- 

 ing cases of total or partial change of 

 the expressed sex of a plant, that is, 

 staminate to pistillate or vice versa, as 

 has been recorded for the Papaya and 

 as has been known to occur to a lesser 

 degree among the willows, sorrel, and 

 some others, there arises a new situa- 

 tion ; under the latter circumstances the 

 abnormal individuals actvially might l)e 

 self-fertile . 



In the class of morphological adapta- 

 tions which make for self-sterility, also 

 would fall all those numberless modifica- 

 ticjns of structure which bring about 

 the prevention of self-pollination. Of 

 course, through the agency of man, 

 dichogamous flowers are capable of 

 being self-poUinated, as they may be 

 also in nature under exceptional cir- 

 cumstances, but as a rule a single flower 

 of either type, when left to itself, would 

 be sterile. 



With respect to heterostyled plants, 



it has been suggested that perhaps the 

 pollen tubes from the short styled 

 flowers were not of sufficient length to 

 penetrate to the eggs of long-styled 

 individuals. It is an interesting sug- 

 gestion but instances in which this has 

 been investigated would not tend to 

 bear it out. Cases are established in 

 which the pollen tubes do not reach 

 the eggs, but this result, so far as deter- 

 mined, is due to a physiological not a 

 morphological cause. 



PHYSIOLOGICAL FACTORS 



Passing now to a consideration of 

 what may be termed the physiological 

 causes of sterility, it will be observed 

 that several classes can be made here, 

 too. The first deals with those pheno- 

 mena classed as chemotactic. This is 

 no doubt a comprehensive grouping, 

 and as more exact data become avail- 

 able, it may be shown that chemotaxis 

 is but a temporarily assigned cause for 

 results not now wholly understood. 

 However, such a classification aids in 

 furnishing a working basis for a con- 

 sideration of available data. 



The greater mass of evidence would 

 indicate that in many instances, it 

 is largely through chemotaxis that the 

 pollen tube is directed in its growi:h to 

 the embryo sac, and the sperms to the 

 egg. Cases of self sterility which can 

 be explained as having resulted from 

 what may be termed negative chemo- 

 taxis are not unknown. To such a 

 cause it seems preferable to ascribe 

 such instances as have been recorded, 

 in which, instead of a progrcs.sive in- 

 crease in the ra])idity of the growth of 

 the pollen tube down the style, there is 

 a progressive retardation, and finally 

 total cessation of growth, but neither 

 the conductive nor stigmatic tissues 

 have been destroyed or perceptibly 

 changed. 



In his experiments \vith orchids, 

 Fritz Miillcr found that not only was 

 the jxillcn of a given plant, when placed 

 u]xjn its own stigma, pre\'ented from 

 germination, l)ut also that there was a 

 poisonous interaction of pollen and 

 stigma. 



To a third class, and so far as orchard 

 fruits are concerned, by all odds the 



