70 THE TISSUES OP PLANTS 



neglected. As childhood ripens into youth, earthy matter 

 is deposited in the bones, they acquire rigidity and firmness, 

 and the extremities straighten. In manhood, their strength 

 has arrived at its maximum development, and they are 

 most admirably adapted, not only to support the softer parts 

 of the body, but to serve as a basis for the attachment of 

 the muscles which execute its movements. As we advance 

 in years, the vitality of the bones diminishes, and in old 

 age they become extremely brittle, owing to the great pre- 

 ponderance of the phosphate of lime over the gelatine. 



The blood vessels of animals appear to be formed like 

 the ducts of plants, by the coalescence of cells arranged in 

 a linear series. In both the ducts and blood-vessels, the 

 organs remain permanently open ; but, whilst the flow of the 

 nutrient fluid speedily subsides in the one, it continues in 

 the other as long as life remains. Again, all the difi'erent 

 varieties of vasiform tissue or ducts, are distinguished by 

 their want of any tendency to branch or anastomose. It is 

 the function of these ducts to convey fluids and air in the 

 most direct manner possible, from ^one extremity of the 

 plant to the other, and therefore they run in parallel lines 

 through the stem and its branches; and, as they are sur- 

 rounded by a sheath of woody fibre, they can have little or 

 no lateral communication until they enter the leaves, where 

 the woody fibre is spread horizontally. But the organiza- 

 tion of the animal body is such, as to require that the blood- 

 vessels should be distributed from the very first on the 

 ramifying principle. Accordingly, those which convey 

 blood to any part, progressively divide and subdivide, 

 diverging from each other like the branches of a tree from 

 its trunk, and their several divisions divide and subdivide, 

 again forming branchlets ; this process of division and sub- 

 division is continued, until the arterial tubes finally ter- 



