252 



PRINCIPLES OF PLANT-TERATOLOGY. 



which are laterally -placed (the usual case), there are- 

 four types of change in their position : (a) From 

 Opposite-decussate to Whorled; (b) from Opposite- 

 decussate to Scattered (Spiral) ; (c) from Scattered 

 (Spiral) to Opposite-decussate ; (d) from Dorsiventral 

 to Radial Symmetry of the Shoot. Terminal leaves 

 are next described, of which there are two kinds, viz., 

 those formed of a single leaf, and those formed of a 

 double leaf. Some cases of the transposition of Bracts 

 are mentioned. 



In the discussion of the subject of phyllotaxis, the 

 spiral type is set forth as the primitive one, the best 

 foundation for which view is afforded by the phyton- 

 theory of Graudichaud, Schultz-Schultzenstein, Cela- 

 kovsky, and others, according to which each leaf- 

 blade, morphologically and primitively, terminates the 

 whole growth of the shoot, the succeeding one sprout- 

 ing from it in a lateral position. The Monocotyle- 

 donous seedling and stem exhibit the primitive type, 

 which is seen also in some Dicotyledons. The single 

 cotyledon of the former is ancestral, from which the 

 two cotyledons of the latter class have been derived 

 by division. The leaf -arrangement in the vegetative 

 stem of plants generally is apparently the result of 

 the mean of two kinds of influence, viz., that emanating 

 from above, representative of the more primitive type, 

 and that coming from below in the cotyledonary 

 region ; the former influence makes for spiral, the 

 latter for opposite phyllotaxis. 



The origin of opposite leaves, normally or abnor- 

 mally, is probably by two methods : (1) By fission of 

 the single leaf at the node, just as in the case of the 

 formation of the two cotyledons in Dicotyledons ; (2) 

 by the extreme shortening and extinction of alternate 

 internodes. Alternate leaves also probably arise, 

 normally or abnormally, in two ways : (1) By fusion 

 of the two leaves of a pair ; (2) by displacement of 

 these. Forked leaves are due either to division (by 

 far the commonest) or fusion, according to the char- 



