DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



with one pair of lateral leaflets in 30 per cent, this simplest type of pin- 

 nation now becoming the modal form of leaf. 



In the fifth leaf (Plate III, 5) the percentages of all the types of single 

 blades had considerably decreased and the pinnate had increased, so that 

 now only 4.3 per cent were 3-lobed and the same number half 3-lobed, 

 17 per cent were unlobed or irregular, and more.than 71 per cent had 

 one pair of lateral leaflets, while a little more than 2 per cent had two 

 pairs of leaflets. 



The sixth leaf was 3-lobed in only 3.5 per cent of the specimens ex- 

 amined and the same number were unlobed or irregular. About 7 per 

 cent were half 3-lobed, nearly 45 per cent had one pair of leaflets, and 

 over 41 per cent had two pairs of leaflets. 



In the seventh leaf none were observed which had not at least one 

 pair of leaflets, though it is not improbable that single blades may occur 

 occasionally in the seventh leaf. The specimens in hand showed 27.5 

 per cent with one pair of leaflets, over 51 per cent with two pairs of leaf- 

 lets, and 21.6 per cent with three pairs of leaflets, so that in the seventh 

 nepionic leaf the form with two pairs of lateral leaflets is the modal 

 form. 



The eighth leaf presents the same three forms, but only a little over 

 9 per cent had a single pair of lateral leaflets, 18 per cent had two pairs, 

 and nearly 73 per cent had three pairs. 



For more ready comparison these percentages are brought together 

 in the following table. The modal form (empirical) in each leaf is 

 given in full-faced type. The variation of leaf-form occurring in each 

 succeeding nepionic leaf from the second to the seventh is shown in 

 Plate III. The left and right sides of the figure show the extreme 

 range of form found in each leaf. The index figures at the left-hand 

 side of the plate indicate the position of the leaves with reference to the 

 cotyledons. 



