AI2 JOURNAL OF SCIENCE. 
trace every gradation, from expanded petiole-sheaths to perfectly leaf- 
like stipules, and this is itself a strong argument in favour of this 
theory of stipular development. In this connection, it is curious to 
note that in certain plants, and particularly among Australian wattles, 
there is a tendency to complete suppression of the true leaves, and 
their replacement by dilated leaf-stalks. The wattle which I have 
here has no leaves, but only these so-called phyllodes. I have seen no 
explanation of this singular phenomenon. I shall speak of the cover- 
ing leaves which take part in the formation of the flower, at a later 
stage in this course. 
A third function acquired by leaves is the storage of reserve food 
materials, but in this the stem which bears them often takes a prin- 
cipal part. Probably all thick leaves have been developed for the 
purpose of storing up reserve materials in the form of starch, oil, &c., 
while seaside plants and those growing in arid regions likewise retain 
considerable quantities of water. In some cases, as in the onion, the 
upper part of the leaf acts as the assimilating organ, while the lower 
part forms a thickened scale-like store of nourishment. When 
thickened bases of leaves thus become closely folded upon one another 
they form what is familiarly known as a bulb, and this bulb is not a 
root as most people imagine, but a small portion of stem surrounded 
closely by these thickened leaf-bases, and capable of sending out roots 
from its under-surface. And the object of this bulb formation is very 
patent. We have to remember that the great object in life of plants, 
at any rate, the chief end of their existence, appears to be to 
reproduce their kind, and this act of reproduction is a most exhaustive 
process. Far instance, the Agaves, which are so much grown in 
gardens for their ornamental appearance, appear to expend all their 
existence in preparation for this act of reproduction. They flower 
just once in their lives, and after ripening their seeds die. The 
onion, like many other plants, has acquired a normal existence of two 
years. The first year’s growth is devoted to the formation of a bulb, 
and the storage in it of a supply of nutritive matter; during the 
second year it flowers, and in forming its seeds uses up the store thus 
laid aside. The process is extremely familiar to us in other garden 
biennials, such as turnip, carrot, &c., in which this function of storage 
has been chiefly taken up by the roots. And lastly, in a large section 
of plants, including many very favourite garden flowering plants, the 
formation of a bulb, which is often just a modified and—as it were—a 
concentrated bud, has become a normal mode of reproduction. 
A more singular function which has been acquired by the leaves 
of a few highly specialised plants is the utilization of animal matter, 
and this has led in several cases to the trapping of living animals. 
One mode of doing this occurs in pitcher plants. These plants are 
not confined to one genus, but belong to several distinct genera and 
orders, and occur in the tropics of the Old and New Worlds, and also 
in the temperate parts of North America. In most cases the pitchers 
are formed from the dilated leaf-stalks, the true leaves being either 
wanting or modified to form covers or lids to the pitchers. I have 
here a figure of Sarracenia purpurea, a native of the northern United 
States. The leaf-stalks are expanded so as to form deep cups, which 
are usually half-filled with water, while the blade forms an open 
winged margin to the cup. Probably much of this water is only rain, 
