402 



GENERAL BOTANY 



Biologically considerable interest attaches to these beautiful 

 plants on account of the unusual devices which result in cross- 

 pollination by insects. Fig. 252 illustrates the usual habit of 

 the violet plants and the highly modified floral structure. In 

 Fig. 253 a sectional view of the flower of Viola pinnata is also 

 shown, which will enable the student to understand more fully 

 the wonderful mechanism for securing pollination and fertiliza- 

 tion in this species. The lower petal of the flower is projected 



backward in the form 

 of a spur, into which 

 two nectaries grow as 

 continuations of the 

 filaments of the two 

 lowest anthers. The 

 stamens form a cone 

 around the style, which 

 is closed except on the 

 lower side between the 

 two lowest anthers. 

 The style projects be- 

 yond the stamen cone, 

 bearing the stigma at 

 its end. When an insect 

 probes down the cavity 

 of the spur for nectar 

 from the nectaries, he spreads the lower anthers slightly, which 

 allows pollen, already dehisced into the anther cone, to sift out 

 upon his proboscis. This pollen cannot reach the stigmatic sur- 

 face of the same flower upon the withdrawal of the proboscis, on 

 account of its position and structure. When, however, the next 

 flower is visited for nectar, the stigma is almost certain to be 

 dusted with foreign pollen gained from a previously visited flower, 

 and so to be either close-pollinated or cross-pollinated. The 

 fruit is a pod which opens by three valves, and the seeds are often 

 thrown to considerable distances by a special mechanism. The 

 two walls of each valve contract as they become dry, and squeeze 

 the hard seeds, which are thus thrown from the mother plant. 



FIG. 253. Structure of the flower of a violet 

 with reference to pollination 



Adapted from Knuth's "Handbook of 

 Flower Pollination " 



