346 Tricotyloiis Races. 



If the variants are rare, they are as a rule normal 

 tricotyls; but if they are more numerous the type is 

 usually seen to be varialjle both in the minus and in tlie 

 plus direction. For convenience of expression we may 

 regard a tricotylous seedling as having arisen Ijy the 

 doubling of one of the two cotyledons of a dicotyledon 

 by splitting, just as is so frequently observed in foliage 

 leaves. Smaller degrees of the splitting would lead to 

 variations in the niinus direction; but if the splitting 

 affects both cotyledons there arise variations in the plus 

 direction, which, if the doubling is complete, result in 

 the origin of tetracotyls (Fig. 63 D). A seedling with 

 one normal and one split seed-leaf is called a hemi- 

 tricotyl •} one with two split seed-leaves, or with three 

 of which one is split, is called a hemi-tetracotyl. In the 

 same way hemi-pentacotyls, and so on, may be found ; 

 but the deviations become rarer as they are more remote 

 from the pure tricotylous type. 



If we make a collection of all these forms it is easy 

 to construct a continuous series which extends from the 

 pure type, on one side, through stages characterized by 

 more or less deep fission, to the dicotyls ; and in the other 

 direction in a similar manner to the tetracotyls and, if 

 the material is extensive, even further, to the pentacotyls, 

 and so on. Fig. 64 exhibits such a series derived from 

 Oenothera hirfella, the unsplit seed-leaf of each plantlet 

 being omitted. But obviously even here the forms fig- 

 ured are only a selection from a much more complete 

 series. If we imagine those cotyledons which have been 

 cut off in these figures to be split also, the series would 

 represent the transition from the tricotyls to the tetra- 

 cotyls. 



^Berirhfe d. d. hot. Gcs., 1894, Vol. XTT, p. 26. 



