Chap, ii.] OF GROWTH IN THE VEGETAL KINGDOM. 281 



an indispeusable part is filled by the atmosphere in the life of 

 organised beings. It is evident that in an atmosphere unsuitable 

 for the maintenance of Jife, or in a vacuum, growth is arrested, 

 the floral and foliaceous buds no longer develop themselves, as 

 Saussure and many other experimentalists have moreover shown. 



The principal facts and general conditions of vegetal growth 

 being determined, we can now describe or point out a certain 

 number of particularities connected therewith. 



The epoch of blossoming often coincides with a more or less 

 rapid acceleration of growth, and, at that time, we see, in the 

 greenhouses, agaves lengthen by more than two decimetres in 

 twenty four hours. The €orypha umbraculifera has been seen to 

 grow forty-five times more during the four months preceding its 

 blooming than it had done in the same lapse of time in thirty- 

 five years.^ Mushrooms grow with extreme rapidity. In three or 

 four days the Lycoperdon giganteum develops into a sphere three 

 dScimfetres in diameter. Certain algse, for example the conf ervae, 

 formed of a single row of cells juxtaposed, end to end, lengthen 

 almost visibly by cellular division. 



In the most complex vegetals, growth has been closely studied. 

 It is produced by the descending and elaborated sap. In the 

 arborescent dicotyledons, the duramen, the ligneous portion, is 

 no longer traversed by the descending sap ; moreover, the fibres 

 which compose this ligneous portion are half mineralized ; they 

 have ceased to multiply themselves, and even to grow. It is 

 between the wood and the bark that the flow of descending sap 

 finds an easy passage, and it is there, in fact, that every year a 

 layer of fresh tissue is formed. Most botanists admit that the 

 creation of the new histological elements takes place by simple 

 division of the old, especially of those which constitute the inner- 

 most layer of the bark, since, according to the experiments of 

 Duhamel, a shred of bark, only connected with the rest by its 

 upper part, or else separated from the aubier by a plate of pewter, 

 still effused cambium on its inner surface. According to M. 

 ' Tieviranus, Siologie. 



