Take another example: The broad, flat, elliptical, green masses so characteristic of the cactus family, and usually called their leaves, are indeed _analogous_ to leaves in color, form, and function; for they are green and flat, and assimilate carbonic acid and water (CO2 and H2O) like leaves. But they are not, in truth, leaves, but modified stems, for they have the essential structure of stems, with their pith, wood, medullary rays, and bark, and may be traced through all gradations into the ordinary cylindrical form of stems. Where are their leaves, then? Their spines are their abortive leaves. These are arranged spirally like leaves, and bear buds in their axils like leaves. They are, in truth, leaves, modified to perform the function of defensive armor; while their function has been delegated to the stem flattened for this purpose. [Illustration: FIG. 3.--A branch of young acacia, showing change from one form of leaf to the other; _a_, _b_, _c_, _d_, successive stages of change; _l_, _s_, leaf stalk which gradually changes into the blade in _c_, _d_, and _e_. ] One more example: The acacias, of which there are fifteen to twenty species in California, introduced from Australia, form two groups having extremely different styles of leaves. We will call them the feather-leaved and the simple-leaved acacias. In the former, the leaves are very finely bipinnate, and the general aspect of the foliage is extremely feathery and graceful. In the latter the leaves are simple, ovate, and, curiously enough, set on edge; and the general aspect of the tree is therefore rather stiff. It seems at first incredible that leaves so different and aspects so diverse should belong to plants of the same genus. But a little close examination shows that, as usual, the botanists are right and the popular judgment wrong. The plumose-leaf is the normal leaf-form for this genus. The simple leaf is not only abnormal, but in a homological sense is not a leaf at all--i. e., it does not correspond to the part called the _blade_ in ordinary simple leaves of other trees. In the seedling of the simple-leaved acacias, and sometimes for a considerable time in the young tree, the leaves are all plumose. As the tree matures it gradually changes its dress and puts on its _toga virilis_. The gradual change from the one form to the other may easily be traced in the same tree, and even often in the same branch (Fig. 3). The steps of the change (_a_, _b_, _c_, and _d_) are shown in the following figure, drawn from nature. It is seen, by bare inspection of the figure, that the so-called leaf, _d_, of the simple-leaved acacias, is really the vertically-expanded leaf-stalk, _l_, _s_, the true leaf or blade being wholly aborted. The whole structure of this so-called leaf is different from that of a true blade. For example, its style of ribbing is parallel, its position is edgewise to the sky, its palisade cells are on both sides alike, etc.