38 MR ALEXANDER MORTON ON 
metamorphosed into spines. There can be no doubt that 
the plants which have thus become suited to such arid 
surroundings are derived from forms which possessed leaves. 
For such indigenous plants as Mereurialis perennis and 
Luphorbia helioscopia have an abundant foliage, and yet how 
different they appear when they are compared with their 
spiny, leafless allies of tropical uplands. 
An interesting problem is presented by the structures 
known as phylloclades. These “ leaf-branches ” are seen in 
many plants belonging to different genera. They are well 
known in the indigenous Ruscus aculeatus. But they are 
characteristic of several other genera in which vestigial 
leaves have been mentioned, as Opuntia, species of Genista, 
Phyllanthus (Kuphorbiaceze), and Phyllocladus (Conifers). In 
many families of these above-mentioned genera, there ‘is 
a tendency to suppress the true leaves, and a relative 
tendency to develop on the twig or branch the green 
assimilating tissue which would naturally have formed the 
leaf, had there been one. The phylloclade represents a 
further development of this nature. It is as if the plant- 
bearing phylloclades found it necessary to revert, to a certain 
extent, to the original form of the organ represented by 
foliage leaves. But since these, in the families mentioned, 
have become so aborted as to be functionally useless, the 
branch bearing these vestigial leaves has adapted itself to 
perform these functions by laterally extending itself. 
In those plants which have become completely parasitic, 
the necessity for leaves altogether disappears. Yet vestiges 
of leaves are found on the bird’s-nest orchid, while no trace 
of them is found on the dodder. The ancestral forms of 
these plants undoubtedly had green leaves, for the parasitic 
habit is the result of a gradual adaptation. It would be a 
matter of extreme interest were it possible to trace all the 
steps in such a process as that by which plants with green 
leaves have become completely degraded to the parasitic con- 
dition. Perhaps the most remarkable feature in parasitic 
phanerogams is seen in the fully developed flowers, a fact 
which emphasises the reproductive as distinguished from the 
vegetative aspect of the life-history. 
The study of the morphology of floral organs furnishes 
