THE AMERICAN BOTANIST 117 



Fall of the Leaf. — Nearly everybody seems to think that 

 the fall of the leaf is due to frost, forgetting in their reason- 

 ing that many leaves fall before frost has touched them. 

 Moreover, if the leaf fall were due to frost we would expect 

 the plants of warm climates to retain their leaves indefinitely 

 which is not the case. The most potent cause of the deciduous 

 condition in plants is undoubtedly drouth, but there are 

 indications that the casting of the leaves may also be due to 

 more deeply underlying physiological conditions. These in- 

 dications show most clearly in the behavior of tropical plants 

 where all kinds of variations in leaf fall may be observed. 

 Some species cast their leaves one by one throughout the year, 

 but the majority have a definite foliar periodicity. The time 

 at which the leaves are cast, however, differs with the species 

 and occasionally with the individuals, or even with different 

 parts of the same individual. In some woody plants one may 

 find specimens in which some branches are clothed with new 

 leaves, others from which the leaves are falling and still others 

 which are leafless. Individuals of the same species of dif- 

 ferent ages may behave differently in this matter. 



The Fuller's Teasel. — Without doubt, the name of ful- 

 ler's teasel (Dipsaciis fiiUonuin) is much more familiar to 

 botanists than the thing itself. Even the "'common" teasel 

 (Dipsacits sylvestris) appears to belie its name in certain 

 regions for the latest edition of "Gray's Manual" makes it 

 "rather rare." Along the main lines of travel in the Northern 

 States, however, the latter species is often encountered, but 

 the fuller's teasel is certainly among the rarities. It may be 

 doubted whether one botanist in fifty has ever seen it growing. 

 In view of this fact, it may be surprising to many to learn that 

 this plant is cultivated in some parts of the United States in 

 quantity, and that in 1910 nearly $7000 worth were exported 

 to Great Britain. That country uses some $75000 worth in 

 the course of a year. The fuller's teasel gets its name from its 



