THE THRKE FOKJIS Or LTTURrSr SALICAEIA. 191 



the flowers are visited by insects, wiiieli would occasiojially bring 

 IJoUeu from other ilowers of the same or of any adjoining plant, 

 as surely as occurs witli the short-styled L. salicaria, of which 

 the pistil and corresponding stamens closely resemble those of 

 L. Jiijssopifolia. According to Vaucher and Leco(i*, this species, 

 ■n-hich is an annual, generally grows almost solitarily, whereas the 

 three preceding species are social ; and this alone would almost 

 have convinced me that L. liijssopifolia cannot be dimorphic, as 

 such plants cannot habitually live by themselves any better than 

 one sex of a dioecious species. 



Ncsaea verticillata. — I raised a number of plants from seed sent 

 me by Professor Asa Grray, and they presented three forms. These 

 differed from each other in the proportional lengths of their organs 

 of fructification and in all respects in very nearly the same way as 

 the three forms of LytTirum Grwffcri. The green pollen-grains from 

 the longest stamens, measured along their greater axis and not 

 distended with water, were j-^jnj- of an inch in lengtli ; those from 

 the stamens ot middle length Yi>^%'> ^^^'^ those {com. the sliortcst 

 stamens ififoxj of an inch. 



We have seen that the genus Li/thrum affords trimorphic, dimor- 

 phic, and monomorphic species. 



The inquiry naturally arises, why do these species differ so 

 remarkably in their sexual relations ? of what service can reci- 

 procal dimorphism or trimorphism be to certain species, whilst 

 other species of the same genus present, like the great majority of 

 plants, only one form ? I have elsewhere given too brieiiyt the 



* Geograph. Bot. de rEurope, tom. vi. (1857) p. 157. 



t ' Origin of Species,' 3rd edit., p. 101. Hugo von Mold has recently (Bot. 

 Zeituug, 1863, S. 309, 321), in a most interesting paper, advanced the case of the 

 minute, imperfectly developed, closed and self-fertile flowers borne by Viola, 

 Oxalis, Impatiens, Campanula, &c., as an argument against my doctrine that no 

 species is self-fertilized for perpetuity. I may state that in the spring of 1862 I 

 examined some of tliese flowers, and saw, tliough less thoroughly, all that 

 H. von Mobl has so well described. I can add only one remark, which I 

 believe is correct, that in V. canina there is an open channel for the pollen- 

 tubes from the extremity of the stigma to the ovarium ; for I gently pressed a 

 minute bubble of air repeatedly backwards and forwards from end to end. 

 Though the imperfectly developed and the perfect flowers are so different in 

 structure, it is a rather cm-ious case of correlation, that in the double purple 

 Violet (r. odorata) the minute imperfect flowers are double to the very core, 

 so that a section appears like the head of a cabbage when cut through. There 

 can be, as vou Mold asserts, no doubt tliat these flowers are always self-fertilized ; 

 they are moreover specially adapted for this end, as may be seen in the remark- 

 able difference in the shape of the pi^til in V. caiuna (and in a less degree iu 

 V. hirla and the single V. odorata) as compared with that of the perfect flower ; 



IINJT. PEOC. — BOT.VNi' VOL. Ylir, 4 



