6 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



(f) Less than 2 per cent had one pair of lateral leaflets, and may 

 perhaps be looked upon as an extreme form of the 3-lobed condition of 

 category (c), but (c) is essentially palmate, while (/) is essentially 

 pinnate. 



A glance at Plate II will show, better than description, the variability 

 of the first nepionic leaf, and it will also show how truly gratuitous is 

 the division into the categories just described. The leaves were chosen 

 at random from the material at hand, but when more leaves were chosen 

 than could find place in the plate the less dubious forms w^ere discarded 

 in order to show the real difficulties of such a classification and to give 

 a clue to the personal equation of the writer in distinguishing the several 

 categories. The letters A, B, C, etc., illustrate the categories described 

 above under (a), (b), (c), etc. 



The first two classes, (a) and (b), are characteristic of the first nepi- 

 onic leaf. With the exception of these two forms, all the categories de- 

 scribed above are represented in each succeeding leaf up to the sixth, 

 and perhaps occasionally in the seventh, and in these later leaves the 

 various types are much more definitely distinguishable, so that there is 

 rarely any question as to where any leaf should be classed. With rare 

 exceptions every leaf after the sixth is pinnate and shows a marked 

 contrast to the great variability of the first leaf, so that in the eighth leaf 

 a very large majority have three pairs of lateral leaflets which closely 

 resemble the typical stem-leaves of the species, except in texture and in 

 the more ovate form of the terminal leaflet. 



In the second nepionic leaf (Plate 111,2) trilobation was found in 

 only a little over 15 per cent of the seedlings, or about one-third as fre- 

 quently as in the first nepionic leaf, but the half 3-lobed form had in- 

 creased from 15 per cent to 18 per cent, the unlobed or irregularly lobed 

 form had increased from 25 per cent to over 61 per cent, and the form 

 with one pair of lateral leaflets from 1.6 per cent to about 5 per cent. 

 Just as the 3-lobed form was the modal condition of the first nepionic 

 leaf, the unlobed form was the modal condition of the second. 



The third nepionic leaf (Plate III, 3) showed a partial return to the 

 3-lobed condition, and nearly 24 per cent of the specimens were so char- 

 acterized. The half 3-lobed form occurred in over 22 per cent, the un- 

 lobed or irregularly lobed form in 39 per cent, and the number having a 

 pair of lateral leaflets had increased from 5 per cent in the second leaf 

 to over 15 per cent in the third. Here, although there was an increase 

 in each of three categories at the expense of the unlobed form, the latter 

 was still the modal form of leaf. 



The fourth leaf (Plate III, 4) was 3-lobed in 24 per cent, half 3-lobed 

 in 15 per cent, unlobed or irregular in nearly 23 per cent, and pinnate 



