526 BOTANY 



case insect visits are prevented. It usually happens that such flowers 

 are at first incapable of self-pollination, owing either to the pistil and 

 stamens maturing at different times, or to their relative positions, so 

 that usually cross-fertilization is effected. Should this early stage 

 pass without insect visits, it may happen that the pollen is deposited 

 upon the stigma of the same flower by a change in the position of 

 stamens and pistil, or by some other contrivance. Thus in the 

 Prickly-poppy (Argemone) the pollen falls from the stamens into 

 the concavity of the widely spread petals. When the flower closes 

 at night, the pollen is deposited upon the stigma, insuring fertiliza- 

 tion in case insects have failed to visit the flower during the day. 

 The same thing takes place in other widely expanded flowers which 

 close at night. 



Special inconspicuous flowers which never open, and are strictly 

 autogamous, are found in a number of plants which also produce 

 showy flowers. These Cleistogamous flowers are formed abundantly 

 in the Jewel-weed (Impatiens fulva) and in various species of Violets, 

 as well as other plants. 



Protection of Pollen against Moisture 



Except in the case of a few submersed aquatics, like Xaias and 

 Zostera, the pollen is quickly destroyed if it is placed in water, 

 as the dense contents absorb water so quickly that the osmotic pressure 

 bursts the pollen. It is therefore important that the pollen should 

 be protected against wetting by rain or dew. This accounts for the 

 drooping position of many flowers, and the overarching of the stamens 

 by the petals which occurs in many others. Probably the closing 

 of flowers at night and in cloudy weather is a provision against 

 wetting, and in a few cases, like the Laurel, the pollen-sacs open by 

 lids, which close when the stamen is moistened, and thus guard the 

 ^enclosed pollen against wetting. 



Protection against Animals 



There are many devices by which plants are protected against the 

 attacks of -animals. These attacks may be in the way of robbing 

 the plant of nectar or pollen, without being useful in pollination, or 

 they may be attacks by animals which feed upon the leaves and 

 stems of the plant. Of the first kind are the visits of small creeping 

 insects, like ants, or the attacks of slugs and snails. The visits of 

 the former may be prevented by the development of sticky glandular 

 hairs upon the stalks and outer leaves of the flowers, a very common 

 device in many plants, such as species of Catch-fly (Silene), Tomato, 

 Petunia, and other " clammy-pubescent " plants. It is possible that 



