THE HIGHER DICOTYLEDONS. 373 



it ; note that the head does not open completely, but remains 

 half closed, as is also the case on dull days. Regular alter- 

 nation of light and darkness is essential ; for the regular 

 opening and closing of the heads. 



How are the opening and closing movements affected by 

 changes in temperature ? Pick a head which has closed for 

 the night, set it in a bottle of water, and bring into a warm 

 room : a temperature of 15 C. or even more will fail to cause 

 opening. A Daisy head when closed in the evening does not 

 readily open with even a considerable rise of temperature ; 

 when open in the morning it does not close unless the tem- 

 perature is greatly lowered. If a closed head is warmed in 

 the morning it readily opens ; if an open head is cooled in 

 the evening it readily closes. 



Note the elongation of the convex top of the flowering- 

 axis after the flowers have been fertilised, and the flat akenes 

 without a pappus. The compressed akenes are doubtless 

 carried by the wind, though very poorly adapted for wind- 

 dispersal as compared with Dandelion fruits. However, in 

 comparing the equipment of plants for the struggle for 

 existence, one must not fix on any single biological advan- 

 tage, but take into consideration the whole equipment. The 

 akenes of Daisy cannot be so often carried to a distance as 

 those of Dandelion, but once a Daisy plant establishes itself 

 in a suitable place it may, by its runners, give rise to 

 hundreds of plants in a few years. 



371. Coltsfoot (Tussilago farfara) is a well-known plant 

 whose flowering shoots appear early in spring (sometimes as 

 early as December), long before the large leaves. The plants 

 grow chiefly in damp heavy soils, in waste places or road- 

 sides, in clayey fields usually in well-exposed situations. 



Note the yellow flower-heads (1 to 1J ins. diameter), each 

 carried on a stem (4 ins. to 1 ft. high) which arises direct 

 from the ground and at first droops but later becomes 

 straight. The flowering-stem bears numerous scales which 

 are, like the stem itself, covered with woolly hairs. At the 

 top of the stem, just below the head, there are several short 

 scales, then a series of long narrow bracts surrounding the 

 flowers. Are the flowers all alike, with tubular or strap- 

 shaped corollas? How many ray-flowers are there? How 



