i868— 1SS1] BLOOM .13 



and arc sure of our facts, I shall ask you whether you know Letta 740 

 of any case of the same leaf differing histologically on the two 



sides, for Hooker always says you arc a wonderful man 

 knowing what has been made out. 



The biological meaning of the curious stru< ture of the leaves of 

 Trifolium resupinatum remains a riddle. The stomata and (speal 

 from memory) die trichomes differ on the two halves of the lateral 

 leaflets. 



To L. Errera. Letter 741 



Professor L. Errer.i, of Brussels wrote, as a student, to Darwin, asking 

 permission to send the MS. of an essay by his friend S. Gevaert and 

 himself on cross and self-fertilisation, and which was afterwards published 

 in the Bull. Soc. Bot. Belg., XVII., 1878. The terms xenogamy, geitono- 

 gamy, and autogamy were first suggested by Kerner in 1876; their 

 definition will be found at p. 9 of Ogle's translation of Kerner's Flowers 

 and their Unbidden Guests, 1878. In xenogamy the pollen comes from 

 another plant ; in geitonogamy from anothcryV^r.v.r on the same flint ; 

 in autogamy from the andicecium of the fertilised flower. Allogamy 

 embraces xenogamy and geitonogamy. 



Down, Oct. 4th. 1S77. 



I have now read your MS. The whole has interested me 

 greatly, and is very clearly written. I wish that I had used 

 some such terms as autogamy, xenogamy, etc. ... I entirely 

 agree with you on the a priori probability of geitonogamy 

 being more advantageous than autogamy ; and I cannot 

 remember having ever expressed a belief that autogamy, 

 as a general rule, was better than geitonogamy ; but the 

 cases recorded by me seem too strong not to make me 

 suspect that there was some unknown advantage in auto- 

 gamy. In one place 1 I insert the caution "if this be really 

 the case," which you quote. I shall be v< ryglad to be proved 

 to be altogether in error on this point. 



Accept my thanks for pointing out the bad erratum at 



1 See Cross and Self-Fertilisation, pp. 352, 386. The phrase referred 

 to occurs in both passages ; that on p. 386 is as follows : " We have also 

 seen reason to suspect that self-fertilisation is in some peculiar manner 

 beneficial to certain plants ; but if this be really the case, the benefit thus 

 derived is far more than counterbalanced by a cross with a fresh stock or 

 with a slightly different variety." Errera and Gevaert conclude (pp. 79-S0) 

 that the balance of the available evidence is in favour of the belief that 

 geitonogamy is intermediate, in effectiveness, between autogamy and 

 xenogamy. 



