590 
MR. F. 0. BOWER OX THE COMPARATIVE MORPHOLOGY OF THE 
of early extension; this check, which the lower pinnae suffer at this early period, 
is permanent, and they appear on the mature leaf as the spines above described. 
It is clear that we have here a further example of a phenomenon noted long ago 
by A. de St. Hilaire,* and called by him “ halancement d’organes,” and which has 
long been recognised by botanists. This mutual dependence of organs one upon 
another in respect of their individual development has been recently again described 
and experimented upon by Goebel (Bot. Zeit., 1880, p. 809, &c.), and designated 
“ correlation of growth.” This author has demonstrated that there is a mutual 
dependence in the mode and extent of their development, between the main shoot 
and lateral shoots, between the foliage- and scale-leaves of a single shoot, and further 
between the sporangia and the vegetative development of the shoot which bears 
them : a stronger development of the former structure in each of these cases being 
shown in a series of examples to be accompanied by a reduction of the latter, while 
if, artificially or naturally, the former structure be removed or reduced, at an early 
stage of development, that removal or reduction is followed by a correspondingly 
stronger development of the latter structures. He further pointed out that this corre¬ 
lation has a very wide influence upon the conformation of the members of plants. 
In the leaf of Cycas Jenkinsiana the correlation may be followed a step further, and 
be recognised as prevailing between the parts of a single leaf. The pinnae situated 
upon that part of the phyllopodium where growth and extension begin at an early 
period are arrested in their growth and develop as spines, while those borne by that 
part of the phyllopodium which extends only at a later period are not so arrested, 
but grow into normal, winged pinnae. A similar correlation appears to me to 
afford at least a partial explanation of the very rudimentary condition of the winged 
structures on the petiole in Cycads, and in many Ferns. Probably a correlation such 
as this is very wfidely spread among plants, and especially in those with petiolate 
leaves. A more exact study of these, and especially of the distribution of their 
growth, not in space only, but also in time, would doubtless lead to an explanation of 
many familiar phenomena of form. 
Dioon edule. 
From the arrangement of parts at the apex of the leaf of Dioon , it appeared not 
improbable that that genus would approach Cycas more closely in the order of origin 
of the pinnae than others of the Enceplialartece. The plant on which observations 
were made bore one mature foliage-leaf, which had on each side about 50 pinnae. 
Both scale- and foliage-leaves in Dioon eclule have a comparatively narrow base, but 
immediately above the point of insertion the margin curves suddenly in a lateral 
direction, so that the basal portion of the leaf is broadly sheathing. The wings of 
this sheath may be traced upwards, as in the leaf of Cycas, to the apex of the phyllo¬ 
podium. As in Cycas the pinnae appear on those ridges, and in leaves in which they 
* Lemons de Botanique, p. 226, 1840. 
