3S6 ECOLOGY. 



the same, but the parts of the flower are in fives, instead of in fours as In 

 the bluet. While the pollen of the short-styleil primulas sometimes must 

 fall on the stigma of the same flower, Darwin has found that such pollen is 



Fig- -(54- 

 Dichogamous flowers of primula. 



not so potent on the stigma of its own flower as on that of another, an ad- 

 ditional provision which tends to necessitate cross pollination. 



In the case of some varieties of pear trees, as the bartlett, it has been 

 found that the flowers remain largely sterile not only to their own pollen, or 

 pollen of the flowers on the same tree, but to all flowers of that variety. 

 However, they become fertile if cross pollinated from a dift'erent variety of 

 pear. 



655. Pollination of the skunk's cabbage. — In many other flowers cross 

 pollination is brought about through the agency of insects, where there is a 

 difference in time of the maturing of the stamens and pistils of the same 

 flower. The skunk's cabbage (Sphathyema foetida), though repulsive on 

 account of its fetid odor, is nevertheless a ver\' interesting plant to study for 

 several reasons. Early in the spring, before the leaves appear, and in many 

 cases as soon as the frost is out of the hard ground, the hooked beak of the 

 large fleshy spathe of this plant pushes its way through the soil. 



If we cut away one side of the spathe as shown in fig. 4^6 we shall have 

 the flowering spadix brought closely to view. In this spadix the pistil of 

 each crowded flower has pushed its st)-le through between the plates of 

 armor formed by the converging ends of the sepals, and stands out alone 

 with the brush-like stigma ready for pollination, while the stamens of all the 

 flowers of this spadix are yet hidden beneath. The insects which pass from 

 the spadix of one plant to another «'ill, in crawling oyer the projecting 

 stigmas, rub off siime of the pollen which has been caught while visiting a 

 plant where the stamens are scattering their ptiUen. In this wa)' cross pollin- 

 ation is brought about. Such flowers, in which the stigma is prepared 



