592 
MR. F. 0. BOWER ON THE COMPARATIVE MORPHOLOGY OF THE 
winged sheath. The pinnae frequently have marginal teeth, which vary in number 
and size, not only from plant to plant, but also from leaf to leaf. While one plant 
observed had hardly any teeth on any of its pinnae, most of them had several cn each 
pinna, the number varying from two to four or five : thus the occurrence of these 
teeth is a very inconstant phenomenon. 
Turning to the process of development, the young leaf of M. Miqueli has a similar 
form to that described for other Cycads (Plate 39, fig. 33), viz. : it is conical, with 
the ventral face flattened, and it is traversed from base to apex by two lateral ridges, 
which are extended near the base as sheathing wings. In their upper part, i.e., above 
the wings, they are at first smooth ; as the leaf grows older, undulations appear first 
close to the apex ; these undulations, which are the first indications of the young 
pinnae, round themselves off as hemispherical papillae ; later they assume an ellipsoidal 
form .(Plate 39, figs. 34, 35). As to the order of succession in the appearance of the 
pinnae it can be clearly proved in this plant that it is basipetal, the same arguments 
being applicable here as in Dioon edule. But there is this difference between the 
two, that whereas in Dioon there are traces of an acropetal order of succession 
in the pinnae nearest the apex, such a succession is completely absent in Macro- 
zamici. It will be obvious from a comparison of figs. 33 to 35 that though the 
leaf may at first increase in length by apical growth, this ceases soon after the 
formation of the first pinnae, and the subsequent elongation must be intercalary : 
also it may be readily proved by measurements that this intercalary growth is 
more active in the lower part ( e.g ., below the fifth highest pinnae in the figs.) than 
in the upper part of the leaf: a similar observation will apply to other examples 
described below. 
The basipetal succession is also maintained by the pinnae in their further develop¬ 
ment : those situated near to the apex of the leaf are the first to assume the ellipsoidal 
form above mentioned (Plate 39, fig. 35) : the longer axis of the ellipse is placed 
obliquely to the axis of the phyllopodium, the lower end of the axis pointing outwards : 
this corresponds to that oblique insertion of the pinnae to be observed in the mature 
leaf of so many Cycads (Plate 39, fig. 37). Very soon after assuming the ellipsoidal 
form, the pinnae show traces of a marginal crenation (Plate 39, figs. 35, 36), the number 
of lobes is small, and corresponds to the number of teeth of the pinnae, of which they 
are the first indication : the teeth thus appear at a very early period, before the tissues 
of the pinnae are differentiated, and a considerable time before the ligrtification of the 
elements of the xylem. 
The pinnae in their further development show a localisation of their intercalary 
growth below the teeth (or pinnules, as they may perhaps be called), so that in the 
mature pinna the teeth are situated at or very near to the apex. It lias been 
impossible to find any single lobe constantly in advance of the others, which could in 
any sense be regarded as the apex of the pinna : still, one lobe is frequently more 
prominent than the rest (compare figs.). - 
