LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
587 
leaves. But after the pinnae have made their appearance, differences in the mode of 
development begin to he apparent, and make it necessary to treat of the two series of 
leaves separately. 
c. Foliage-leaves. 
The apical growth of the phyllopodium, which was never very marked, ceases with 
the appearance of the pinnae ; a slow intercalary growth is however maintained through¬ 
out its length, but is specially localised at different points in it at different times. In 
the basal portion the transverse intercalary growth is continued slowly throughout the 
transverse section, producing a widening of the sheath, which thus accommodates itself 
to the increasing bulk of the younger leaves within it; this transverse growth is accom¬ 
panied by a slow longitudinal growth resulting in an elongation of the sheath. The 
most active intercalary growth goes on in the part of the phyllopodium above the sheath ; 
this part elongates as the petiole, and carries the part which bears the pinnae up with 
it. Meanwhile the pinnae themselves remain in close juxtaposition one with another, 
the elongation of the part which bears them being subsequent to that of the lower 
part or petiole. From the observation of marks, originally drawn at equal distances 
apart on the young elongating leaf, it may be observed that the point of maximum 
activity of elongation is at first at some distance below the insertion of the pinnge, and 
gradually proceeds upwards along the leaf; the final result is that the ultimate 
extension of the tissues is greatest in the sections immediately below the lowest 
pinnae. 
Meanwhile the petiole has increased in bulk, and presents the well-known almost 
cylindrical form, but the traces of the two longitudinal ridges may still be found in 
their original positions, and are well marked in certain other species (e.g., C. revoluta). 
Passing on now to the pinnce, as above stated, these soon assume the form of rounded 
papillae which show in section that they consist of a mass of meristem, covered by a well- 
marked dermatogen. These papillae elongate transversely to the axis of the phyllopodium, 
but their growth is not specially localised, though there is a slight indication of this at 
the base of each pinna. The growth is unequal and hyponastic, resulting in the well- 
known circinate curvature of the young pinna. The tissues at the apex of the young 
pinna remain actively meristematic after two complete circles of circulation have been 
completed, and even after the pinna has begun to uncurl; but the activity at or near that 
point is not greater than elsewhere. Soon after this the apex passes over to the 
condition of permanent tissue, and as it does not take part in the subsequent develop¬ 
ment of wings, it remains in the mature leaf as a distinctly acuminate process. 
The rounded form of the young pinna is soon lost by the formation of lateral wings 
similar in relative position to those on the phyllopodium. A transverse section of a 
very young pinna shows an almost circular outline, while the tissues appear 
undifferentiated, and all the cells capable of division. Later the longitudinal divisions, 
i.e., in planes parallel to the organic axis of the pinna, cease in the central portion of 
4 F 2 
