944 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
these two layers are also reversed. This inversion can be more 
or less decidedly brought about by compelling certain leaves 
to develop their surfaces reversed. Moreover, when leaves 
assume a vertical position, so as to be equally illuminated on both 
sides, then the epidermis and subjacent palisadic tissue are also 
more or less alike, as in some grasses, the phyllodes of Australian 
Acacias, the pendulous falcate leaves of gum-trees, though the 
first-formed horizontal leaves on the same gum-trees are dorsi- 
ventral in structure like ordinary horizontal leaves *. 
The preceding facts all conspire to prove that the differen- 
tiation of the mesophyll into palisadic tissue is the direct result 
of illumination; and that, as the upper surface of normally 
horizontal leaves receives more light than the lower, the differen- 
tiation of the chlorophyll-cells into an elongated form parallel to 
the incident light is the direct outcome, ceteris paribus, of that 
external agent. 
A significant fact with regard to these alterations is that 
the change evinces itself by degrees. Thus in young leaves 
of Alstremeria psittacina, M. Dufour ¢ shows that it is only 
partially effected. Thus the first leaf stands vertically. It 
possesses few stomata, and is nearly alike on both sides. The 
second leaf is still nearly vertical, being only twisted towards 
the apex. It has stomata on both faces at the base, but at the 
point shows many upon the upper, but none on the lower side. 
The subsequent leaves have the petiole twisted and are com- 
pletely reversed in position ; their petioles have a small number 
of stomata on the two faces in the limb, exclusively upon the 
* The chief differences between the two forms of leaves in Eucalyptus I find 
to be as follow :—In the horizontal leaf the upper epidermis is composed of 
small cells and there are no stomata. There is a palisade-tissue of one layer of 
cells, with lax mesophyll below the lower epidermis. ‘This latter has larger 
cells than the upper and is provided with stomata. The pendulous leaf is a 
good deal thicker than the horizontal. Both epidermides are provided with a 
very dense cuticle in which the stomata are deep-seated. There are four rows 
of palisade-cells on both sides with a chlorophyllous mesophyll between them. 
The petiole is flattened so that the leaf can swing much in the same way as that 
of the Poplar. The horizontal leaves in E. Globulus are sessile. 
M. G. Briosi has written a voluminous work with many plates upon the 
leaves of Eucalyptus Globulus, Labil., to which the reader is referred for 
numerous details (‘ Intorno alla Anatomia delle foglie dell’ Eucalyptus Globulus, 
Labill.,’ 95 p. et 23 pl., Milano, 1891). 
t Bull. Soc. Bot. de Fr., 23 mai, 1886, p. 269. The author here gives several 
references to the literature of the subject. 
