274 THE WONDER OF LIFE 



its early life as a semi-parasite inside the gill-cavity of the 

 freshwater mussel. 



Bees and Flowers. — The inter-relations between bees 

 and flowers have formed the subject of many studies and of 

 many controversies. For the matter is not so clear and 

 simple as is sometimes represented. Bees visit the flowers 

 for the pollen and the nectar. The cane-sugar of the nectar 

 is transformed into glucose and is consumed as food by its 

 collector, or is stored in cells. The pollen serves as food 

 directly, or it is mixed with honey to form a nutritive 

 paste or jelly for the young. In hive-bees there is often a 

 good deal of method in the collecting; Aristotle noted 

 rightly that they often keep to one kind of flower at a time. 

 There is often division of labour among the workers, for 

 some collect nectar and others collect pollen. The adapta- 

 tions on the bees' part are many, but the most important 

 are the suctorial mouth-parts and the pollen-collecting 

 hairs on the legs. 



The egg-cell of a flowering plant hidden away withui 

 the ovule within the ovary does not usually develop into 

 an embryo unless it be fertilized by a male element (nucleus) 

 within the pollen grain. The pollen graias are dusted on 

 to the stigma of the pistil in various ways — usually by 

 insects or by the wind or by shaking — and from a poUen 

 grain a pollen-tube grows down in search of the egg-csU. 

 It is a nucleus withia the pollen-tube that effects the f ertfli- 

 zation proper and sets development agoing. Unless this 

 happens, the ovules or possible seeds do not become real 

 seeds containing embryos. 



Now it is well-known that although sel£-fertihzation 

 occurs (e.g. in peas), cross-fertihzation is predominant. 

 That is to say, fertilization is usually effected by pollen 



