THE THREE FORMS OF LYTHRUM SALICARIA. 193 
This result would appear to be one of high importance, for with 
dimorphie plants it is ensured at the risk of occasional sterility ; 
not only is the pollen of each plant useless or nearly useless to 
that individual, but so is the pollen of all the plants of the same 
form, that is, of half the total number of individual plants. In 
that extensive class of plants called by C. K. Sprengel dicho- 
gams, in which the pollen of each flower is shed before its own 
stigma is ready, or in which the stigma (though this case occurs 
more rarely) is mature before the flower’s own pollen is ready 
sterility can hardly fail to be the occasional result; and it would 
be the inevitable result with both dichogamous and reciprocally 
dimorphic flowers unless pollen were carried by insects (and in 
some few species by the wind) from one flower or plant to the 
other. As with reciprocal dimorphism so with dichogamy, 
within the same genus some of the species are and some are not 
thus characterized. Again, in the same genus, as in that of 
Trifolium, some species absolutely require insect-aid to produce 
seed, others are fertile without any such aid ; now when insects are 
requisite for fertilization, pollen will generally be carried from one 
flower to the other. We thus see, by means of reciprocal dimor- 
phism, of dichogamy, and of insect-aid, that some species require, 
or at least receive, incessant crosses with other individuals of the 
same species; whereas other species of the same genera can be, 
and probably are often fertilized during long periods by the 
pollen of their own flowers. Why this wide difference in the 
frequency of crosses should occur we are profoundly ignorant. I 
will only further remark on this head, that it would be a great 
mistake to suppose that many flowers, which are neither reci- 
procally dimorphic nor dichogamous, nor require insect-aid for 
their fertilization, nor show any particular adaptation in their 
structure for the visits of insects, are not habitually crossed with 
the pollen of other individuals; this occurs, for instance, habitually 
with cabbages, radishes, and onions, which nevertheless are per- 
fectly fertile (as I know by trial) with their own pollen without 
aid of any kind. 
* But it may be further asked, granting that reciprocal dimor- 
phism is of service by ensuring at each generation a cross (but I 
am far from pretending that it may not have some additional 
unknown signification), why did not dimorphism suffice for 
L. salicaria and Grefferi? why were they rendered reciprocally 
trimorphie, entailing such complieated sexual relations? We 
cannot answer, except perhaps so far :—if we suppose two plants of 
EK 
