THE LEAF. 



219 



by a very accentuated torsion of the stem. This is due 

 to the fact that while the leaves composing each pair 

 tend to become separated owing to the stretching of 

 the stem, their bases really remain united, and this 

 occurring along a length of stem including many 

 internodes, inevitably causes the torsion phenomenon. 

 PI. XXI shows an excellent example of this in the 

 malmaison carnation (Dianthux Caryophyllus var.). 

 It is also frequently seen in the teasel (Dijpsacus 

 sylvedris) (fig. 56) and many other plants with 

 opposite-decussate or whorled leaves. Not only, how- 



b 



Fig. 56. — Dipsacus stjlvestris. Ground plan of normal shoot (a) and 

 of shoot showing- transition to spiral phyllotaxis (b). (After De 

 Vries.) 



ever, do the leaves of the same whorl remain united, 

 but all the leaves of the shoot, from the base upwards, 

 become congenitally united at the base, a fact which 

 seems common to all plants exhibiting this torsion. 

 This, then, is another instance of the rare phenomenon 

 of leaf -fusion in the vegetable kingdom. 



There is another, much rarer, method by which the 

 transition from opposite-decussate to alternate phyllo- 

 taxis is effected, viz. by lateral fusion, at one or more 

 nodes, of the opposed leaves of a pair ; Alex. BroAvn 

 observed this in the first pair of foliage-leaves suc- 

 ceeding the cotyledons which, fusing, formed a single 

 leaf alternating with the first leaf of the alternate 



