282 BIOLOGY. [BOOK in.. 



Tre*cul, there is probably also an advent of new elements by 

 spontaneous genesis. 1 In the opinion of this observer, when the 

 buds are produced, there is also an effusion of gelatiniform 

 blastema between the bark and the aubier. This effusion forms 

 a mamelon ; then in this mamelon appear little cells. M. Ch. 

 Robin goes farther. According to him differentiated histological 

 elements spring forth also by genesis in the centre of the cellulous 

 mamelon. We should observe the appearance of ovoid cells 

 arranged in a single cluster, and having from the beginning a 

 reticulated aspect ; then afterwards cells with a spiral thread 

 spring forth. When once the evolution of the bud is more 

 advanced, when once the leaves are formed, the primitive, single 

 cluster ramifies to send little clusters into each leaf. 2 



The pith grows energetically when it is not imprisoned in a 

 rigid ligneous tissue, which isolates it from the cortical system. 

 In this last case, on the contrary, it often dies, is destroyed, and 

 the stem becomes fistulous. But when the pith is bound by 

 transverse radii to the bark and to the cambium, it most fre- 

 quently grows by cellular division. Notwithstanding, Dutrochet 

 has affirmed that he saw new cells spring forth, by spontaneous 

 genesis, in the cellular interstices of the pith. The medullary 

 cells are generally soft, and very osmotic ; they readily imbibe 

 water or sap, become turgescent, and fit to produce generative 

 blastemas. 



The duramen is a kind of senile state. Trees which are desti- 

 tute of it, as the poplar and the maple, grow generally with 

 greater rapidity than others ; for growth then takes place 

 throughout the whole thickness of the trunk, and not only 

 between the bark and the aubier. In fact, there is growth 

 wherever the elaborated sap penetrates. Dutrochet cites, as an 

 example, the radiciform stem of the beetroot, composed of layers 

 of loose cellular tissue, between which the cambium circulates. 3 



1 Trecul, Anncdes des Sciences Naturelles, 1846. 



2 Ch. Robin, loc. cit., p. 39. 



3 LOG. cit., t. II. p. 162. 



