PERTINACITY AND PREDOMINANCE OF WEEDS. 241 



element of their predominance. Apparently this question 

 must be answered in the negative. The question is not 

 whether they are self-fertilizable. The great majority of 

 plants are so, even of those specially adapted for intercross- 

 ing. The plants of this list appear to belong to the juste mi- 

 lieu. Only one (Mumex Acetosella) is completely dioecious ; 

 a few are incompletely dioecious or polygamous ; the two spe- 

 cies of Plantago are dichogamous to the extent of necessary 

 dioicism or monoicism ; a large number of the corolline spe- 

 cies are either proterandrous or proterogynous, including two 

 or three anemophilous species ; and all the Grasses (which 

 form the last quarter of the list) are anemophilous and more 

 or less dichogamous, and therefore not rarely cross-fertilized. 

 Of those which are not anemophilous we notice none which 

 are not habitually visited by insects (except perhaps Gnapha- 

 lium uligino sum), and which therefore are' almost as likely to 

 be cross-fertilized as close-fertilized ; while in not a few (such 

 as the Compositce generally and most of the other Garnope- 

 talce) the arrangements which favor intercrossing are explicit. 

 There is no cleistogamous and therefore necessarily self -fertil- 

 ized plant in the list, except Lamiuum amplexicaule, which 

 also cross-fertilizes freely. 



In California the prevalent weeds are largely different from 

 those of the Atlantic States, and, as would be expected, are 

 mostly of indigenous species or immigrants from South Amer- 

 ica ; yet the common weeds of the Old World, especially of 

 southern Europe, are coming in. The well-established and 

 aggressive ones, such as Brassica nigra, Silene Gallica, Ero- 

 dium cicutarium, Malva borealis, Medicago denticulata, Mar- 

 rubium vulgare, and Avena sterilis, were perhaps introduced 

 by way of western South America. They are mostly plants 

 capable of self-fertilization, but also with adaptations (of dicho- 

 gamy and otherwise) which must secure occasional crossing. 



We cannot avoid the conclusion that self-fertilization is 

 neither the cause nor a perceptible cause of the prepotency 

 of the European plants which are weeds in North America. 



A cursory examination brings us to a similar conclusion as 

 respects the indigenous weeds of the Atlantic States, those 



