Flowers 



is the number of ray florets in the heads of various 

 composites. It has been found that the American Aster 

 prenanthoides varies in this respect according to the 

 season. Early in the year, the average number of ray 

 florets is small ; they reach a maximum in summer, and 

 then gradually decrease again. But towards the end 

 of the season there is often a slight increase in the 

 average number of ray florets. 18 



So that there really is a variation in flowers when 

 tested by statistics, which is a very important point. 



In a systematic account of the genus Erigeron, it was 

 tried to trace the history of the various changes. In 

 Linosyris, for instance, (i) all the flowers in the head are 

 similar. (2) In the next stage some of the outer florets 

 have lost their stamens. (3) Next these outer florets 

 became threadlike and altered in shape (as e.g. in Conyza), 

 by the petal-tube contracting on the style. (4) Some 

 of these outer florets enlarged their petals and became 

 ray florets (Trimorpha). (5) These attractive florets 

 became sterile, and the inner threadlike florets became 

 female only. 19 



For these changes to have happened, one would like 

 to know how, for instance, the flowers in the second 

 stage lost their stamens. 



But when a kind of savoury (Satureja hortensis) is 

 cultivated in particularly poor soil, it shows a tendency 

 to suppress its stamens. 



As a rule, in this plant 79 per cent, of the flowers 

 possess both stamens and carpels, but in very poor soil 

 this may sink to 17 per cent, or even 13 per cent. 

 Also, at the end of the season, almost all the flowers 

 have no stamens at all. 20 



So such changes as those of Erigeron are not neces- 

 sarily inexplicable, they may be directly connected with 

 the climatic conditions. 



121 



