DAEWIN OK PEIMULA. 237 



7/7000ths of an inch In diameter, while in the short-styled form the 

 stigma is depressed and nearly smooth, the pollen-gi-ahis ranging from 

 10/-to-ll/-7000ths of an inch in diameter. Our own observations 

 entirely confirm the minute accuracy of these statements, though of 

 the relative sizes of the pollen-grains we have only judged by com- 

 parison of them by the eye, on the same slip of glass under the 

 microscope. 



To these differences between the two forms, we may add another, 

 noticed while dissecting the flowers. The ovules of the long-styled 

 Primula, which Mr. Darwin states to produce a smaller number of 

 seeds, are considerably larger (and probably less numerous) even 

 before the flower expands, than in the short-stj^led form, which he 

 finds to produce the larger number of seeds. These two forms — the 

 long-styled and short-styled —occur in nature in about equal pro- 

 portions. It is not yet satisfactorily shown that the same plant can 

 produce both forms, though this is a point to which we think further 

 attention might be directed, especially m those species which have 

 occasionally a second or autumn flowering. 



Now the carefully conducted experiments of Mr. Darwin, which 

 are described in detail in his paper, show a remarkable difference in 

 the influence exercised by the pollen upon the stigma of its own 

 flower and upon the stigma of a flower of the other form. Fertilisa- 

 tion of a flower by pollen of its own form he terms ' homomorphic,' 

 by the pollen of the other form ' heteromorphic' And it is the hete- 

 romorphic unions which he shows to be pre-eminently fertile. If, 

 therefore, the abundant production of good seed be advantageous to 

 the species, so must be heteromorphic fertilisation, a process de- 

 pendent however upon circumstances, which we may call accidental, 

 though they are nevertheless certain and ever-acting. The agency ot 

 insects is absolutely necessary for the crossing of the different forms, 

 and there can be no question but that the part they play in this 

 economy is of the very highest importance. Having explained the 

 provision which nature has made to favour the crossing of distinct 

 individuals, Mr. Darwin suggests the possibility that the species of 

 Primula may possibly be tending to a dioicous condition. In their 

 present condition they are, as he observes, ' subdioicous hermaphro- 

 dites.' "We are not in possession of corresponding facts relative to 

 any other species in either of the groups which are distinguishable of 

 dimorphic flowers ; so that, unable to institute a single comparison, we 

 are reduced to the necessity of specidating upon very meagre mate- 

 rials. AV^e have referred the case of the Primulas to one category 

 with imisexual or diclinous flowers, whether of monoicous or dioicous 

 plants does not immediately affect the question. We have done so 

 simply because between the comparatively trivial amount of dielinism 

 in Primula and the more extreme instances which are at hand on 

 every side in overwhelming number, we are utterly unable to draw 

 the line. 



Before we proceed to give a few instances from our second 



