Fig. 5.—A, long-styled, and B, short-styled, forms of _Primula veris_. C, D, E, the three forms of the trimorphic flower of _Lythrum salicaria_, with petals and calyx removed on near side. (After Darwin.)] Amongst plants there are some very interesting cases of dimorphism and trimorphism in the structure of the flowers. Darwin has studied some of these cases with great care, and has made out some important points in regard to their powers of cross-fertilization.[30] The common European cowslip, _Primula veris_, var. _officinalis_, is found under two forms, Figure 5 A and B, which are about equally abundant. In one the style is long so that the stigma borne on its end comes to the top of the tube of the corolla. The stamens in this form stand about halfway up the tube. This is called the long-styled form. The other kind, known as the short-styled form, has a style only half as long as the tube of the corolla, and the stamens are attached around the upper end of the tube near its opening. In other words, the position of the end of the style (the stigma) and that of the stamens is exactly reversed in the two forms. The corolla is also somewhat differently shaped in the two forms, and the expanded part of the tube above the stamens is larger in the long-styled than in the short-styled form. Another difference is found in the stigma, which is globular in the long-styled, and depressed on its top in the short-styled, form. The papillæ on the former are twice as long as those on the short-styled form. The most important difference is found in the size of the pollen grains. These are larger in the long-styled form, being in the two cases in the proportion of 100 to 67. The shape of the grains is also different. Furthermore, the long-styled form tends to flower before the other kind, but the short-styled form produces more seeds. The ovules in the long-styled form, even when unfertilized, are considerably larger than those of the short-styled, and this, Darwin suggests, may be connected with the fact that fewer seeds are produced, since there is less room for them. The important point for our present consideration is that intermediate forms do not exist, although there are fluctuating variations about the two types. Moreover, the two kinds of flowers never appear on the same plant. Darwin tried the effect of fertilizing the long-styled flowers with the pollen from the same flower or from other long-styled flowers. Unions of this sort he calls illegitimate, for reasons that will appear later. He also fertilized the long-styled flowers with pollen from short-styled forms. A union of this sort is called legitimate. Conversely, the short-styled forms were fertilized with their own pollen or with that from another short-styled form. This is also an illegitimate union. Short-styled forms fertilized with pollen from long-styled forms give again legitimate unions. Footnote 30: