^5ouml\ aIX?i^^^^^ Structure of Compound Leaves. 217 
Y. — On the Simple Structure of Compound Leaves. 
By W. K. McNab, M.D., Edin. 
The object of this paper is to point out that many of the leaves 
which Mr. Gorham compares together, to show the transition from 
the compound to the simple leaf, do not admit of comparison. In 
comparing the parts of such leaves as the Rosa canina and the- 
Oriental Plane, although they seem to present great similarities 
when placed side by side, yet a knowledge of their development 
shows that the parts compared are not of the same value. The 
same may be said of the Horse-chestnut and Sycamore. 
There is no morphological distinction between simple and com- 
pound leaves. They both arise from the phylloblast in precisely 
the same manner ; and, as the parts of the first order are developing, 
it is impossible to say whether serration, or lobes, or pinnee will 
result. The difference between a simple and a compound leaf is 
thus not one of type, but a difference in the degree to which the 
division of parts is carried. The so-called metamorphosed leaves of 
Mr. G-orham are arrests of development — the differentiation of parts 
not having been carried to its fullest extent. 
If we examine the development of the phylloblast of Bosa 
canina, we find that the upper part, or epiphyll, which develops 
into the lamina and petiole of ordinary leaves, exhibits that type of 
development which we find common in the Bosacem, caUed the 
Basipetal Type. 
The central pinna of this impari-pinnate leaf is the oldest ; then 
the upper pair ; while the lower pair nearest the axis is the youngest. 
These parts are all of the first order — the serration forming the 
divisions of the second order. In the Kose the parts of the second 
order develop basipetally. 
Contrast this with the Oriental Plane. Here the leaf develops 
according to the ternate type. Three parts of the first order are 
developed ; the side lobes are parts of the second order, and pro- 
duced by bifurcation of the side parts of the first order. 
I have not examined the development of the Virginian-creeper, 
so cannot say to what type it belongs ; but I suspect it belongs to 
the Basipetal type. It may therefore admit of comparison with the 
leaf of the Kose ; but, as all the parts of a leaf of the Oriental Plane 
do not belong to the same order as those of the Eose, they do not 
admit of a comparison being drawn between them. 
The leaf of the Sycamore might be compared with that of the 
Eose : — the parts of the Sycamore-leaf develop basipetally, and are 
therefore of the same value as those of the Eose ; but the parts of 
the second order, the serrations, develop in the reverse way, and are 
