Prof. C. Morren on the discoid Piths of Plants. 81 



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 be- 

 comes of a clear yellow, by the drying up of the membranes 

 of the cells, and briUiant points are formed ; these are nume- 

 rous crystals which originate from the diminution of the hquid 

 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 sHt, 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 ap- 

 pearance. 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 crystallized in different forms. This is 

 the period of death. 



What we have just proved in the case of the Begonia, we 

 are able to see going on, with some few modifications, in 

 Juglans regia, which offers several facts worthy of remark, and 

 of which we shall speak briefly. 



The greater part of the buds of this tree are supported on 

 short branches ; in this case the pith which is compact and 

 without cavities is also very short, so that that which is formed 

 of discs rises very high in the branch (fig. 6.). But it is by no 

 means necessary to stop till the end of the first year, as M. 

 DeCandoUe has said, to see this compact pith converted into 

 disciferous pith : it happens in the first year, and that at a 

 very early period. The dissection, fig. 6 bis, clearly shows 

 that the pith in the vicinity of the buds is quite full of juices, 

 and that its separation into discs begins in the middle en 



