SECT. II 



PHANEROGAMIA 



499 



a poorer yield of seed (Kye) or be without result (self-sterility in 

 Cytisus Laburnum, Lohelia fiilgens, Corydalis Cava, some Cruciferae, etc.). 

 It may even have a directly injurious influence, as in certain orchids 

 in which the application of the Hower's own pollen causes the flower 

 to die. In certain plants in addition to the large CHASMOGAMOUS flowers, 

 pollinated by wind or insects, small inconspicuous flowers occur which 

 never open and only serve for self-fertilisation ; these cleistogamous 

 flowers (") afford a further means of propagating the plant, while the 



Fig. 454. — Inflorescence of Margravia wmbellata adapted for pollination by Humming-birds. 

 (From SoHiMPER, Plant Geography.) 



plants have the opportunity of occasional cross-pollination owing to 

 the presence of the large chasmogamous flowers. 



Cleistogamous flowers are the result of an arrested development, though their 

 sexual organs become mature. The pollen-sacs usually lack the mechanism for 

 opening, so that the pollen-tubes have to grow through the wall of the anther to 

 reach the stigma. The occurrence of cleistogamous flowers is in all cases trace- 

 able to insufiicient nutrition, whether due to lack of mineral food-materials or to 

 insufficient light. Since seed is regularly set by these flowers, the plants bearing 

 them can succeed even when chasmogamous flowers are not developed. Cleistogamy 

 is of frequent or regular occurrence in species of Impaticns, Viola, Lamium, and 

 Stcllaria, in Specularia perfoliata, Juncus bufonius, etc. FoJycarpon tetraj)hylluvi 

 lias only cleistogamous flowers. 



On the other hand there are many and various conditions which 



