Prof. C. Morren on the discoid Piths of Plants. 77 



exhibited a continuous pith in the first and an interrupted pith 

 in the second. 



On the section of an internode where the lenticular cavities 

 are formed we see first the derm (a, fig. 2.), then the cellular 

 envelope (b, fig. 2.), after which comes the ligneous sheath 

 (c, fig. 2.), which at the nodes abuts on diaphragms that are 

 also ligneous but not formed of fibres (d, fig. 2.). In the 

 centre appears the pith (e, fig. 2.) and its cavities (f, fig. 2.). 

 These cavities do not in their first state extend to the exterior 

 limits of the pith, and their first appearance is that of a sim- 

 ple transversal slit. These slits are separated by portions of 

 compact pith, which are of six, eight, or ten times the extent 

 of the slits. 



These slits, are they lacerations of tissue ? So we should 

 believe, according to the assertion of M. DeCandolle. But see- 

 ing their evenness, the outline of their separation broken off all 

 at once, and the smooth and polished surface of their sides, 

 this became very improbable. Where there are rents of tissue 

 there is raggedness, irregular indentations, and jagged frag- 

 ments ; here there are none of these. I presumed there- 

 fore that this w T as rather a separation of tissue, and a sepa- 

 ration brought about by a particular disposition of the ele- 

 mentary parts. Examination with a microscope proves to me 

 in fact that nature had so contrived it. 



Figure 4. explains this structure. The pith is formed by 

 cells of 8-sided prisms, the sections of which are hexagons, 

 but these hexagons are elongated and all laid transversely, 

 so that the pith is definitely formed by layers of horizontally 

 elongated cells. 



Now the slits are simple separations of these layers, with- 

 out the cells themselves being in any way affected (I, m, fig. 4.). 

 I am well aware, that on examination of piths cut longitu- 

 dinally for the purpose of seeing these slits, open cells will 

 be found, but these arise from the dissection; the others are 

 all perfectly closed, but simply separated one from another. 



Here then is a first fact established, that if the pith is 

 broken or slit, this interruption of continuity arises from a dis- 

 location of the layers of cells, and that these cells are, in order 



