236 REVIEWS. 



therefore, to devote the short space at our disposal to a review of the 

 facts and conclusions established by Mr. Darwin, directing attention 

 to other instances of dimorphism iu other and very different species 

 of flowering plants. 



We may, after a certain fashion, rudely group the kinds of di- 

 morphism exhibited in the flower under two heads. First, a dimor- 

 phism, apparently favourable to variation, marked primarily by a 

 partial or complete separation of the sexes, which may be accom- 

 panied or not by alteration iu the form or arrangement of the 

 parts of the perianth surrounding them ; and, second, a dimorphism, 

 conservative, and unfavourable to variation, marked prunarily by an 

 alteration in the form or arrangement (frequently a reduction) of the 

 outer whorls of the flower, which more or less completely enclose and 

 seal up the sexual organs, which are never wholly separated.* 



Such grouping we may well designate as rude, but there do cer- 

 tainly appear to be two classes or kinds of dimorphism, which even 

 in the present state of our knowledge — feeling as it were our uncertain 

 way — it may be well to distinguish, and we do not see how better to 

 define them thau as above. 



It is to the first group that we may refer the primroses, and with 

 them a very numerous company indeed of trees, and shrubs, and 

 herbs. There are comparatively few natural orders of flowering 

 plants out of the 200 or 300 which are generally recognized, in which 

 we do not find more or less of a diclinous condition — a condition 

 which necessarily involves " dimorphism " in respect of the sexual 

 organs. There are numerous Orders invai'iably, or almost invariably, 

 characterized by luiisexual flowers. There are others again in which 

 a tendency to this condition is more or less conspicuously manifest 

 in many of their members. A large proportion of the trees of tem- 

 perate Europe bear only flowers thus dimorphic. In the oak, beech, 

 chestnut, and pine, for example, this dimorphism is extreme. In the 

 stamen-bearing flowers, we find no rudiment of a pistil — in the pistil- 

 bearing, no rudiment of stamens. But between plants which we 

 may regard as wholly homomorphic, and consequently witli flowers 

 completely hermaphrodite, and the extremes just cited, Ave have an 

 infinity of intermediate conditions. 



Parting from the hypothetical truly homomorphic hermaphrodite, 

 we find in the case of Sir. Darwin's Primulas one of the first grades 

 of incipient dimorphism of which cognizance can be taken. 



Hence one peculiar interest of his observations, to which we recur. 

 Besides the difterences abeady mentioned in the relative length of 

 the style and height of the anthers in the corolla-tube of these plants, 

 Mr. Darwin points out that in the long-styled form the stigma is 

 globular and rough with minute papilla?, and the poUen-grains about 



* This second f!;ronp we have not framed to indnde a dimorphic condition of the 

 male flower, or of the female flower, of a unisexual plant. We arc not aware, how- 

 ever, that such exist. If there he none, the circumstance is worth noticing. 



