190 DISCOID PITH. [BOOK i. 



elaborated fluid of the sap, is more and more subtracted 

 towards the bud, the pith dries more and more, the slits are 

 multiplied and grow so large as to be true lenticular cavities, 

 which leave between them medullary discs. The latter then 

 are formed by layers of cells nicely separated one from 

 another, out of a mass primitively common, but without 

 laceration of the partitions ; these, at first double for the con- 

 tiguous cells, are now become isolated. At the same time 

 the pith loses its green colour and becomes of a clear yellow, 

 by the drying up of the membranes of the cells, and brilliant 

 points are formed ; these are numerous crystals which originate 

 from the diminution of the liquid in which their elements 

 were originally dissolved. It is so true that these changes 

 take place in this manner, that if we cut a stalk of Begonia 

 argyrostigma longitudinally, when it is fresh and the pith 

 only slit, at the end of two days we see the slits become 

 lenticular cavities, and the medullary discs are formed at the 

 same time that the pith drying up passes from green to 

 yellow, and the crystals make their appearance. In fact the 

 sap is lost by evaporation, as in the plant it disappears by 

 the suction of the bud ; but it is lost, and the same causes 



bring about the same results. Fourth period. The bud 



being developed and the branch formed, the pith is become 

 useless. It is deprived of all its juice; its cellular tissue, 

 whose cells are become large, is dried up completely ; the 

 desiccation has separated all the layers of cells, and a 

 considerable number of discs have been formed ; brown dry 

 discs formed by the empty cells, without and within which 

 the salts have crystallised in different forms. This is the 

 period of death." 



The Walnut Tree offered another instance of discoid pith, 

 the gradual formation of which Professor Morren also traced, 

 and thus describes : " The compact pithin the Walnut Tree 

 is composed of a number of small cells nearly in the form of 

 cubes, all equal to one another, white, transparent, having 

 very few globules, but containing at a very early period 

 masses of small crystals, or true muriform calculi, which 

 occupy the centre of the cells. At a later period, when the 

 pith separates into discs, and dies, the cells undergo very 



