BOTANICAL EVOLUTION. ATI 
have just to notice that those plants which develope leaves that. 
float on the surface (like Potamogeton) tend to have them quite 
unbroken.* 
It would be perfectly impossible in the time at our disposal to 
consider the innumerable modifications which leaves have undergone 
in anything like detail, and therefore I shall only refer briefly to them, 
erouping them by their functions, and describing more in detail some 
of the most remarkable developments. The primary function then is 
the assimilation of carbonic acid and the transpiration of water, and 
associated with this simple laboratory work is the synthesis of cellulose, 
wood fibres, protoplasm and its allies, starch, sugar, gum, and the 
thousand and one substances which occur in plants. This primary 
function is still carried on by all ordinary foliage leaves and green 
parts of plants, but the modifications of form, consistence, &c., have 
arisen from secondary causes. The necessity for protection against 
herbivorous animals, whether belonging to the vertebrates or insects, 
has been met in a variety of ways; by spines, as in holly, whin, or 
thistle, by hard consistence only, as in so many shrubs, by hairiness, 
which is so common a feature among our New Zealand alpines, by 
bitter or acrid juices, as in dandelion, buttercups, &c.—and no doubt 
by other devices—as the auctioneers put it—too numerous to mention. 
A second function, which must have been acquired later, is that 
of acting as covering organs; special leaves being in many cases 
produced for the purpose only of protecting others. We see these in 
the scale leaves which are so familiar to us at this time of the year 
covering young leaf buds. You will find them in abundance in 
flowering-currants, lilacs, as well as on many of our introduced 
deciduous trees, such as poplars, willows, oak, &c. In the horse- 
chestnut, every gradation can be traced, from the little scales on the 
outside of the undeveloped bud to the perfect leaves lying within, 
but these leaf scales are always extremely deciduous; that is to say, 
that as soon as the buds they are designed to protect have expanded, 
and their function is thus ended, they drop off. Another kind of 
covering leaf is termed the stipule, and these organs are considered to 
be of great value in a classificatory point of view. Here you have 
stipules well developed in the pansy. Stipules are usually stated to 
be modified foliage leaves, but this explanation has never been satis- 
factory tome. I cannot go into all the grounds of my objections here, 
but will state that in my opinion they are developments of the sheath 
of the leaf itself, and not separate leaves. Here you have for example 
the leaf of a buttercup, and you notice that the petiole or leaf-stalk 
has a sheathing base. Now most plants of the order Ranunculacez, 
to which the buttercup belongs, have the bases of their leaves sheath- 
ing, but none have true stipules. But in their most closely related 
allies, the Magnoliacee, stipules are present, but act as true leaf-scales, 
dropping off before the leaves are fully expanded. Further, we can 
* In ‘‘ Nature” of 26th February appears the abstract of a lecture by Sir John 
Lubbock on the forms of leaves, in which he suggests that ‘‘in perfectly still air 
: . finely-divided leaves may be an advantage, for the same reason that they 
are advantageous in water, namely, that they expose so large a breathing surface, 
whereas in comparatively exposed situations more compact leaves may be more 
suitable.” It was pointed out that finely-cut leaves are common among low herbs, 
and that some families which among the low and herbaceous species have such 
cot m shrubby or ligneous ones have leaves more or less like those of the laurel 
or beech. 
