I3I2 



A TEXTBOOK OF THEORETICAL BOTANY 



which are really adapted for wind pollination and in such cases it is doubtful 

 whether the insect performs any useful service to the flower. 



Some plants are good for both pollen and nectar, e.g., the White and Red 

 Clovers, which are worked by bees for both materials. A bee visits 300-400 

 flowers to collect one load of pollen, which takes from 20-30 minutes, 

 carrying off about 1,000 grains from each flower. 



Pollen flowers are generally white, yellow or red in colour; violet or blue 

 flowers rarely provide a surplus of pollen. 



Some pollen flowers are peculiar in that the stamens are beset with fine 

 hairs, as for example in Verbasciim (Fig. 123 1), where the three posterior 

 stamens are hairy and the anterior pair, which lie close to the stigma and 



provide most of the fertilizing pollen, 

 are naked. The hairs provide a good 

 foot-hold for the visitors, especially 

 Hover Flies, which suck juice from the 

 hairs and eat the pollen of the pos- 

 terior stamens, while the abdomen is 

 dusted with pollen from the anterior 

 stamens. The flowers are incompletely 

 protogynous and self-pollination may 

 occur. 



2. Zygomorphic Flowers 



Ludwig has drawn attention to the 

 peculiarities in certain species of Cassia. 

 The differences in the position and 

 character of the stamens in these flowers 

 can be best explained on the assump- 

 tion that some provide pollen for the 

 visitor while others are concerned with 

 the cross-pollination of the flower. 



In Cassia marylandica (Fig. 1232), a 

 member of the Caesalpinaceae, the 

 flower is visited by Humble Bees. The 

 flowers are zygomorphic, with a calyx 

 composed of five sepals enclosing five 

 free and almost equal petals. The 

 arrangement of the ten stamens is pecu- 

 liar. The three upper ones are much 

 reduced in length and have no anthers. 

 In some species they may be absent, 

 but in this species they may play the 



Fig. 1232. — Cassia warylaudica. A, 

 Young flower with two long, pol- 

 linating stamens in front and 

 shorter " fodder stamens " be- 

 hind. B, Older flower after all 

 pollen has gone, showing recep- 

 tive stigma uncurled. (After Engler.) 



part of nectar guides, though no nec- 

 tar is actually secreted. The four lateral stamens are also short and the 

 anthers open by lateral slits. They produce abundant pollen which is 

 readily gathered by the bees as a source of food. The lower stamens are 



