PROTECTION OF GREEN LEAVES AGAINST ATfACKS OF ANIMALS. 447 



system of spines. Accoi-ding to the onlinary conception of the inetliod of 

 feeding of the herbivorous animals referred to, it would be thought that these 

 green cluiiijis, pillars, and balls, even without their terrible eiiuipiueiit, would 

 form anything but choice food. But when they are seen in their original habitats, 

 it is easily seen that they have every need to protect themselves and to defend 

 their existence. While on the stony and sandy plains and slopes which form 

 the habitat of cactuses, all other plants have long been withered, and a green 

 leaf can no longer be seen for far and wide, when all the springs of water are 

 dried up, and not a drop of rain has moistened the ground for months — then 

 the cactuses still remain always fresh and green, and by the assistance of their 

 central aqueous tissue, they are able to survive through the greatest drought 

 and aridity whicli are ever observed on the earth. But at such period.s of 

 drought, every cactus-ball appears like a cordial to the liungry and thirsty 

 animals, and frequently even as the only alternative to death. In spite of the 

 frightful spines with which species of Melocactus are bristling, these are sought 

 by the wild asses in the plains of South America at the periods of greatest 

 drought, and are rooted up where possible by their hoofs in order to get at the 

 juicy tissue of the unarmed lower parts; or the animals try to split the cacti 

 with their hoofs, and in this way to get at the interior, in vrhich proceeding it 

 very often happens that the assailants injure themselves by the spines and receive 

 dangerous wounds. 



Next to the cactuses, the strangest spine-formations are exhibited by the low 

 half-shrubby tragacanth bushes (Astragali) belonging to the group Tragacanthacei, 

 which, in an inexhaustible variety of species have their habitat throughout Southern 

 Europe, but chiefly in the east, on rocky mountains and elevated steppes. We will 

 pick out one, viz. Astragalus Tragacantha, from the large number of species, and 

 explain by words and picture the remai'kable protective contrivance of its green 

 foliage-leaves (fig. 118 ^). On observing this plant very early in the spring, a tuft 

 of numerous long, dry, grey spines, whose points are directed upwards and outwards, 

 is seen on the free extremity of each branch. In the centre of this tuft of spines 

 lies a bud, which forms the top and termination of the branch in question. The 

 warmth of spring causes this bud to develop, and the close-pressed pinnate leaflets 

 become loosened, stretch out, and unfold; but weeks pass by, and the leaflets are 

 always still surrounded by the bushy garland of spines. Their gi-een colour can 

 only be seen shining through from behind the long spines as from behind the grey 

 lattice bars of a cage. When they are fully developed, and when they have also 

 somewhat lengthened the branch they adorn, the uppermost leaflets at length 

 project beyond the points of the spines. But see — the end leaflet which had been 

 situated on the rachis of the pinnate leaf has already dropped ofi^, and often a 

 pair of the lower leaflets with it (fig. 118=), and all that now projects beyond 

 last year's spines has also become changed into a spine. The rachis of the leaf at 

 the point where the terminal leaflet was formerly inserted becomes hardened and 

 transformed into a pricking point. Then comes autumn, the period of the leaf-fall. 



