FERTILISATION IN FLOWERING PLANTS. 283 



weather ; and in aquatics, either by a peculiar covering and structure 



as in Zostera, or by the flowers being developed above water, as in 



Kymphsea, Lobelia, Stratiotes, and Hottonia. In Vallisneria spiralis 



(fig. 613), a plant growing in ditches in the south of Europe, the stami- 



niferous flowers are detached from the 



male plant, float on the surface of the 



water, and scatter their pollen ; while 



the pistilliferous plant, b, sends up a 



long peduncle, which accommodates 



itself to the depth of the water by 



being spiral, and bears on its summit 



the flower with the pistil. By this 



means the two organs are brought into 



contact, and fertilisation is efiected. 



Lagaxosiphon muscoides, an aquatic 



plant from Africa, shows similar phe- "^ Pig s^, 



nomena in regard to impregnation 



as are seen in Vallisneria. When continued wet weather comes on 



after the pollen has been matured, and has begun to be discharged, it 



often happens that little or no fruit is produced. In flowers where 



the anthers burst in succession, the injury done by moisture is less 



Ukely to extend to all. Stamens are protected in various ways from 

 wind and moisture. In Iris by the petaloid divisions of the style, 

 in Phyteuma by the upper united part of the corolla, in TroUius by 

 the sepals turned inwards, so as to form a ball (hence the name globe- 

 flower), and in Arum by the spathe (flg. 260, p. 178). In many 

 flowers the perianth gives shelter to the stamens. In Orchids the 

 pollen is well protected. 



In some plants the Stamens, at a certain period of their develop- 

 ment, move towards the pistil, before the contents of the anther are 

 discharged. In Parnassia palustris (fig. 514) and Rue they do so in 

 succession. In Kalmia the anthers are contained in little sacs or 

 pouches of the corolla, until the poUen is mature, and when the 

 expansion of the corolla and the elasticity of the filament combine to 

 liberate them, they spring towards the pistil with a jerk. In Parie- 

 taria oflScinalis, and in the Nettle, the spiral filament is kept in a 

 folded state until the perianth expands, and then it rises with elastic 

 force and scatters the pollen. Similar phenomena are observed in the 

 Comus canadensis. In the various species of Barberry the inner and 

 lower part of the filament, is irritable, and when touched it causes 

 the stamen to move towards the pistU. The anther opens by recurved 



Fig. 513. Male and female plants of Vallisneria spiralis, a, The male plant, the 

 flowers of which are detached, and rise to the surface of the water so as to mature 

 its pollen and scatter it ; 6, the female plant, which remains fixed in the mud, and sends up 

 a spiral peduncle, which uncoils according to the depth of the water, and hears the pistil- 

 liferous flowers above the water, so as to allow the pollen to be wafted upon them. 



