278 BIOLOGY. [Book hi. 



diffuse themselves into the substance of the anatomical elements ; 

 living molecules of new formation intercalate themselves by 

 intussusception with the molecules primitively existing; the 

 cellular membrane extends ; the protoplasm grows ; then the 

 form of the histological elements often modifies itself. It is in 

 this manner, for example, that the ligneous fibres form themselves. 

 The cells, at first spherical, elongate, and become cylindrical ; 

 their extremities adhere to each other, either squarely or 

 obliquely. Concentric layers deposit themselves successively on 

 the inner surface of the partition, which they thicken.^ 



In other tissues the spherical or polyhedrical cell gi-ows simply 

 by increasing in age without losing its first form. Sometimes, 

 even, we can determine the relative ages of the cells of a tissue 

 by the variations in volume. In the pith of many plants the 

 cells go on regularly and gradually decreasing in volume from the 

 central region, where they are the most aged, to the circum- 

 ference, where they are youngest. ^The same fact may be 

 observed in comparing the medullary cells of the same stem at 

 different heights. Dutrochet has. observed that in certain plants 

 the gradual decrease of the diameter of the stem from the base 

 upwards is owing solely to the decreasing volume of the medul- 

 lary cells, beginning at the neck of the root.^ In efEect, shoes 

 of the pith of the elder-tree, cut at different heights, contained 

 manifestly the same number of cells. 



In the inferior vegetals, especially in certain families of alg» 

 (conjugated, diatomous, siphonated), there is neither division 

 of labour nor specialisation of tissue. The cell then lends itself 

 to various functions, metamorphoses itself, adapts itself to 

 multifarious uses. Besides, whatever may be the degree of 

 complication of its structure, it generally preserves the faculty 

 of segmenting itself, of multiplying itself. It is quite otherwise 



1 Oh. Robin, Des EUments Anatomiqw^ (BibUomque des Sciences 

 NaturelUs). 



« Dutrochet, Minwirespour Sermr i. VHiatoire Physiologiiue et Anatomiqm 

 dcs Animaux et des Vigitwux, t. II.. p. 139, 



