448 PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



Most deciduous shrubs now throw off the leaves with which they have worked ali 

 the summer, by means of the formation of a layer of separation at the place where 

 the leaf is inserted on the stem, as previously described. But this does not occur in 

 the tragacanth bushes. Only a portion of the long grey spines by which this year's 

 leaves were surrounded are cast off. The leaflets of the present year are now 

 detached from the leaves; the strong midribs or axes whose ends had already 

 become changed into spines during the summer remain firmly joined to the stem 

 and dry up, forming thus a new stiff tuft of spines which is as like the one thrown 

 off as one egg is to another. Accordingly the dried-up remains of the leaves of one 

 year, now changed into spines, become an apparatus for protecting the developing 

 green leaves of the year following. Observation in the natural state shows that 

 these projecting spines are capable of protecting the green leaves behind them from 

 the attacks of grazing animals. One may see how grazing animals stop in front of 

 the shrubs bristling with spines, and actually abstain from further attacks after the 

 first attempt, although the foliage of the tragacanth named, like that of other 

 papilionaceous plants, would furnish a very desirable meal. 



Different from the Tragacanth is the Barberry (Berberis). On looking in 

 summer time at a shoot in vigorous growth, it will be seen to be beset with two 

 kinds of leaves: first, with leaves which have anything but the appearance of 

 foliage, being transformed entirely into spines like those of the cactuses. These at 

 the base of the shoot are drawn out into from five to seven, and further upwards 

 into three, needle-shaped points, as shown in figs. 118^ and 118''. Short branches 

 beset with ordinary green foliage-leaves arise simultaneously in the axils of these 

 metamorphosed leaves. These short branches terminate in buds which develop 

 early in the following year, and then form either flowers, or long branches. The 

 foliage-leaves of the short branches, below these buds, fall off in autumn. The 

 three-pronged spines at the bases of the short branches, i.e. of the buds which have 

 passed through the winter, remain behind, and radiate out from the shoot with 

 their three needles in three directions. Now, when in the following spring the 

 buds at the end of the short branch swell, and young tender foliage-leaves burst 

 from them, these are excellently protected against being devoured as long as the 

 points of the three-pronged spine still project beyond them. 



In Robinia Pseudacacia, popularly known by the name of Acacia, and also in 

 numerous other robinias as well as in several Siberian caraganas {Caragana 

 microphylla and pygmcea), the stipules are transformed into prickles, and not, as 

 in Berberis, the whole leaf. In all Leguminosas, structures arise right and left of 

 the place of insertion of the leaf on the stem, known as stipules {stipul(B), on 

 account of their position. They are not leaf-like in the robinias and shrubs named, 

 but are transformed into brown spines drawn out into a sharp point. When the 

 foliage-leaf becomes detached and falls off in autumn, both the spinous stipules 

 remain behind and persist even on into the following summer. In the axil of the 

 two divergent spine-stipules is situated a bud which unfolds in the following spring. 

 Here we have again repeated the same protective mechanism as was previously 



