214 SCIENCE PROGRESS. 



sexual cells may perhaps often cause colour phenomena 

 as mere "bye-products". The phenomena of chemotaxis, 

 movement in response to chemical stimulus, seem to 

 indicate a chemical difference between male and female 

 cells, and we need not therefore be surprised at a difference 

 in colour between male and female flowers such as is often 

 seen. 



DICHOGAMY. 



When stigma and pollen do not ripen simultaneously 

 the flower is said to be dichogamous ; when they do, it is 

 homogamous. If the stigma be first ripe, the flower is 

 protogynous ; if the pollen, protandrous. The multiplication 

 of observations in various localities goes to show that this 

 phenomenon also is very variable. In some regions a plant 

 may be highly protandrous, in others only slightly so, or it 

 may even be homogamous or protogynous. It has also been 

 observed to vary with the season of the year. 



FLOWERS AND THEIR INSECT VISITORS. 



In this department of the subject an enormous mass of 

 material has been collected. The literature relating to the 

 European flora has all been collected and published in 

 abstract by Loew. The results support the Mullerian 

 theory of flowers very well, and are of interest in showing 

 the great differences in the floral phenomena between 

 different localities, even in plants of the same species. 

 This point we have already considered. 



A commencement (4) has been made upon the British 

 flora, with the result of showing that short-tongued flies 

 perform to a large extent the work done by the smaller 

 bees, etc., upon the continent. MacLeod has worked out 

 the flora of Flanders in a most thorough way, and a good 

 deal of other similar work has been published. 



MacLeod has also investigated the annual development 

 of the chief groups both of flowers and insects in Flanders. 

 For each natural order of flowers the number of species in 

 flower in each month was noted ; the resulting curve ex- 

 hibits in each case a gradual rise to a maximum and a 



