THE HIGHER DICOTYLEDONS. 369 



The Dandelion is a very suitable plant on which to make 

 daily observations with a view to learning the complete life- 

 story of the flower-head and of the individual flowers. By 

 watching the same head at frequent intervals day by day, 

 and by examining flowers of different ages, you can make out 

 a good deal of the story for yourself. Is the head always 

 expanded ? Under what conditions does it close ? Note 

 how the straps of the outer flowers close over the inner 

 flowers, and how the closing is completed by the inner circle 

 of bracts. Does it close at night only ? For how many 

 hours does it remain open each day in fine weather ? Do all 

 the flowers in a head open on the first day that the head 

 itself is expanded ? How often must the head open in order 

 that all the flowers in it may have a chance of being 

 pollinated by insects ? 



In watching Dandelion-heads in a garden I have noticed 

 that, as an average, the heads are only open for about eight 

 hours on fine days and that a head rarely opens more than 

 thrice. In fine weather nearly all the flowers in a head are 

 open by the end of the third day, and on the fourth or 

 fifth day the head is closed and the corollas are beginning 

 to wither, while on cold and wet days the heads remain 

 closed. How do these observations agree with yours ? 



Open up flowers of different ages, and note that at first the 

 style is low down in the tube formed by the united anthers, 

 the two stigniatic arms being in close contact by their inner 

 (stigmatic) surfaces ; the pollen is shed inside the tube, and 

 is swept out of the top by the hairs on the style as the latter 

 grows up. During this "male" stage, insects visiting the 

 head remove the pollen-mass from the top of the anther-tube 

 as it is pushed up by the piston-like style, and since the 

 stigmatic surfaces are not exposed, only cross-pollination is 

 possible. Then the style emerges and its two lobes diverge, 

 exposing the stigmas; if cross-pollination does not occur 

 during this second or "female" stage, the arms curve back 

 until the stigmas touch the pollen still adhering to the style, 

 thus bringing about self-pollination. 



After the corollas have withered and the head has closed up 



for the ripening of the fruits, the head- stalk usually bends 



down towards the ground, and an interesting series of 



changes takes place. What changes do you observe in (1) 



s. B. 24 



