THE ANGIOSPERMAE 



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lowed by the pollen tubes because it offers mechanically the path of least 

 resistance and possibly also the best food supply. The tubes push their way 

 between the cells, chiefly by mechanical pressure, though they may secrete 



Fig. 1 32 1. — Oenothera biennis. Longitudinal section 

 of style showing penetration by pollen tubes. 



a pectase which softens the middle lamellae. Only rarely do they penetrate 

 into cells. Pollen tubes which follow a superficial course are called ecto- 

 tropic and those which follow an intercellular course are called endotropic. 

 The difference was at one time given some systematic importance, but it is too 

 variable for this. The route followed is different in some closely allied 

 genera and even {e.g., Cucurhita) in species of the same genus. The parti- 

 cular direction followed seems to be physiologically determined and the 

 differences argue in favour of a chemotactic influence on the direction of 

 growth. What this may be is not definitely known but it has been found that 

 most proteins are chemotactically positive for pollen tubes and only a few 

 are negative. Among the sugars, glucose and sucrose are sometimes positive 

 but not in all cases, while maltose is indifferent. The attractive substances 

 may therefore be proteins and the importance of the sugars is more probably 

 osmotic. The whole subject is very puzzling. What is the source of the 

 attraction? If it is the placenta, why do the tubes leave the placenta and pass 

 across an air-space to the micropyle, as they often do? If it is the embryo 



