MANUAL OF NATURAL HISTORY. 499 



their growth is unlimited, terminating only with 

 the death of the individual, while among the former 

 it is limited, being arrested at a definite period by a 

 cessation of generative power in the woody cells. 

 Endogens are destitute of true pith, and their stems 

 are seldom hollow, exceptional cases such as in 

 many of the grasses, being caused by the circum- 

 ference growing more rapidly than the centre. The 

 leaves are not reticulated, as among exogens, but 

 are straight-veined, and the parts of the flower are 

 usually arranged in series of threes ; the embryo, 

 too, is composed of but one cotyledon, whence they 

 were, by De Candolle, termed monocotyledones. 

 Their period of vitality, also, is more restricted than 

 it is in the higher class, seldom extending beyond 

 two or three hundred years, though occasional 

 instances of more extended longevity occur, as in a 

 dragon-tree, still existing at Teneriffe, which, more 

 than 450 years ago, was regarded as very aged. 



The number of genera known at present is up- 

 wards of 1,400, which include about 13,600 species. 

 Among these are numerous plants of the greatest 

 importance io man, as various members of the family 

 of grasses, some yielding him abundant farinaceous 

 aliment, while others afford nutriment for his flocks 

 and his herds. The palms supply wine, sugar, 

 edible fruits, and sago j arrow-root is the produce of 

 species of Maranta; the aromatic root of the Zin- 

 giber constitutes ginger ; while turmeric, obtained 

 from Curcuma longa, forms a valuable dye, and is 

 the basis of some admired condiments. The number 



