284 THE FLOWER. 



body, and alternating with these five two-cleft glands, the ver- 

 tical chink or groove of which is glutinous. To each gland is 

 firmly attached, by a caudicle or stalk, a pollen-mass of an ad- 

 jacent anther. (Fig. 522.) A slight force upraising the gland 

 detaches it from the stigma and drags the pair of suspended 

 pollen-masses out of their cells. Insects visiting the blossoms 

 commonly dislodge them, the gland adhering to their legs or 

 tongues when these happen to be drawn through the adhesive 

 chink, and convey them from one flower to another. Without 

 such aid the flowers of Asclepias rarely set seed. 1 



423. Dimorphism, i. e. the case of two kinds of blossoms, both 

 hermaphrodite, on the same species, is another adaptation to 

 intercrossing. Not all dimorphism, however, for in cleistogamous 

 dimorphism (434) the intent to self-fertilize is evident. There 

 may also be dimorphism as to the perianth, not particularly 

 affecting fertilization. One kind, however, and the commonest, 

 is a special adaptation to intercrossing, viz. : 



424. Heterogonous Dimorphism. (413, note.) This term is 

 applied to the case in which a species produces two kinds of 

 hermaphrodite flowers, occupying different individuals, the flowers 

 essentially similar except in the androecium and gyncecium, but 

 these reciprocally different in length or height, and the adapta- 

 tions such that, by the agency of insects, the pollen from the 

 stamens of the one sort reciprocally fertilizes the stigma of the 

 other. 2 This dimorphism has been detected in about forty genera 

 belonging to fourteen or fifteen natural orders, widely scattered 

 through the vegetable kingdom ; but there are far more examples 

 among the Rubiaceae than in any other order. Sometimes all 

 the species of a genus are heterogonous, as in Houstonia, and 



1 The reported sensitiveness of the gland, referred to in the first issue of 

 this volume (1879), was founded upon misinterpreted observations. 



* This peculiar arrangement has been long known in a few plants, such 

 as Primula veris, P. grandiflora, and Houstonia. In Torrey and Gray's 

 Flora of North America, ii. 38, 39 (1843), these flowers are said to be dicecio- 

 dimorphous, not denoting that they are at all unisexual, but that the two 

 forms occupy different individuals. Their meaning was detected by C. 

 Darwin, and made known in his paper " On the Two Forms or Dimorphic 

 Condition in the Species of Primula, and on their Remarkable Sexual Rela- 

 tions," published in the Journal of the Linnean Society, vi. (1862), 77 : repub- 

 lished, in 1877, as the leading chapter of his volume entitled "The Different 

 Forms of Flowers on Plants of the Same Species." Mr. Darwin had termed 

 these flowers simply Dimorphic; but in this volume he adopted Hilde- 

 brand's name of Heterostyled for this kind of blossom. The difference, 

 however, affects the androecium, and even the pollen, as well as the style ; 

 wherefore we proposed for it the name of Heterogonous or Heterogone dimor- 

 phism, as mentioned in a former note, 413. 



