Linnæus, Philos. Bot., p. 215, § 272, says:—
"Spinosae arbores cultura saepius deponunt spinas in hortis." Lothelier found that Barberries grown in a moist atmosphere had no spiny leaves, and that the thorns were far less woody under those conditions, whilst in a perfectly arid and dry atmosphere only spines were formed; a strong light also tended to produce spines.
Professor Sickenberger grew a desert plant (Zilla myagroides) in the Botanic Garden at Cairo, and found that its spines were much weaker and more slender than the strong rigid thorns which cover it in its natural desert.
Professor Henslow[94] found that the spiny form of the Rest Harrow, when grown in a rich soil with an abundance of water, gradually loses its spines. All these experiments certainly show that a dry desert sort of life, and possibly strong sunlight, favour the development of spines and thorns.
Of this there cannot be any reasonable doubt, for the extraordinary quantity of thorny, spiny things in deserts shows that there must be some connexion between such a life and their production (see Chapter X.). In such places animals are always abundant. But these hollies, pears, and other plants show exactly the opposite to what we should expect. It is when the head of the young holly reaches the sunlight and feels the wind that its leaves become harmless!
If one remembers the case of the young larch and its goat enemies on page [181], it is perhaps possible to think that the lower branches and twigs were for untold generations exposed to laceration and biting. Thus, suffering from the loss of water by these regular annual wounds, the leaves developed their spines in response. So far, belief is not more difficult than it is with regard to the origin of any variety. But whenever, by reversion to their ancestral type, the original not-spiny leaves developed on the top of a tree, that tree would have an advantage, for every leaf on it would be more economically produced; a smooth leaf would not require to spend food in order to make spines. Such trees, spiny below and smooth above, would be best fitted to survive, healthier and more vigorous, and in the end would leave more descendants.
At the same time, such a case as this reveals again that mysterious and exquisite purposefulness which a reverent mind discovers in Nature everywhere.
At the same time, as we have already pointed out, we are exceedingly ignorant of many of the very commonest facts. Léo Errera, the great Belgian botanist (whose recent death has been a terrible loss to science), collected together some facts as to the taste of cattle for various spiny and thorny plants; he found that cattle wished to eat the following: Buckthorn, whin or gorse, raspberry, brambles, the Scotch thistle, the creeping thistle, as well as musk, welted and slender thistles, sow thistle, and saltwort.
They avoided: Barberry, the petty and German whin, rest harrow, the carline, and the other thistles not given above, as well as the common juniper.
They disdained or despised: Sea holly, common holly, milk thistle, Lactuca, and Urtica urens.[95]