286 DICHOGAMOUS PLANTS. 



and the pollen is carried from the one to the other, the scheme is 

 perfect. The three forms are divided according to their styles into 

 long-styled, mid-styled, and short-styled. Such plants may be called 

 trioeoiously hermaphrodite. The fertilisation is effected by the agency 

 of insects. The insect in passing from flower to flower will brush 

 against a stigma at a given level with the same part of its head or 

 body which has brushed off the pollen from an anther at a corre- 

 sponding level. The object of all these arrangements is the pre- 

 vention of close inter-breeding. Homomorphic unions, where a pistil is , 

 supplied with pollen from its own flower, or from a flower of the same 

 form, result either in very diminished fertility, or, as in the dimorphic 

 species of Linum (Flax), in absolute sterility. 



The same object — namely, the prevention of close inter-breeding — 

 may be effected by other means ; sometimes, as in Orohidacese (fig. 

 317, p. 205), and Asclepiadacese (figs. 385, 386, p. 230), by the 

 mechanical arrangement of the parts of the flowers, and, more 

 especially, the consistence of the pollen, being such that fertilisation 

 cannot occur without the agency of insects, which carry the pollen 

 masses (poUinia) from one flower to another. In the species of 

 Orchids, such as Orchis mascula, the pollen masses (fig. 387, p. 230) 

 have each a caudicle, which is firmly attached to a viscid disk, con- 

 sisting of a minute oval or rounded piece of membrane, with a ball of 

 viscid matter on its under side. These balls are contained within a 

 cup-like rosteUum, the lip of which is easily depressed by contact with 

 a foreign body, such as the proboscis of an insect. The pollinia be- 

 come thus attached to the proboscis. At first they stand erect, but 

 ultimately, by the contraction of the minute disk, they bend down- 

 wards and forwards towards the point of the proboscis. In this way 

 the pollen is in a position to be at once applied to the stigma when 

 the insect visits another flower, and thus fertilisation is effected. 



The prevention of close inter-breeding is also accomplished in many 

 cases by the physiological condition of the parts concerned in fertilisa- 

 tion, as occurs in what are called Dicho- 

 gamous plants — that is, plants in which 

 the stamens and stigmas of the same flower 

 do not reach maturity at the same time — 

 the stamens being matured first in what 

 are called protandrous plants, and the 

 stigmas first in protogynous plants. (See 

 notice of Protandrous and Protogynous 

 plants, at page 212.) In Pamassia palus- 

 ^^' ^^^ tris (fig. 514) the stamens move in suc- 



Fig. 514. Flower of the Grass of Parnassus {Pa/r7iassia paZustris), the stamens of which 

 move in succession towards the pistil, and discharge their pollen. In the figure some 

 stamens are seen applied to the pistil, and others removed from it. 



