STRUCTURE.] CHAMBERED PITH. 189 



In certain Trees, of which the Walnut is an example, the 

 pith is regularly broken up into small horizontal chambers 

 separated by thin discs. It is not, however, so when young, 

 but the structure belongs to its advanced age. Professor 

 Morren has investigated the history of this curious structure 

 in the Annals of Natural History, vol. iv. The following are 

 extracts from his interesting paper, which deserves to be 

 studied in extenso. He thus describes the changes observed in 



the pith of the Silver-spotted Begonia (B. argyrostigma.) 



" First period. The pith is continuous, full, compact, without 

 interruption of continuity. It is composed of cells which 

 have been spherical, and which are become prismatic by their 

 mutual compression. These cells lengthen by degrees trans- 

 versely, and end by being disposed thus in horizontal planes. 

 At this period the cell is filled with a liquid, and fecule ; it 

 overflows with nutritive substance ; its pith appears green, 



like the germinating cotyledon of a plant. Second period. 



The pith is become more extended by the development of 

 the branch; the fecule changes into alimentary juice; it 

 dissolves by the operation of nutrition (does it become 

 gum?), first disappearing from the central cells of the pith, 

 where the nuclei are formed at the same time with some 

 granules of chlorophylle. By the loss of this nutritive 

 substance, such inorganic substances as salts obey the forces 

 of the inorganic world, and crystallise by degrees ; the crystals 

 being formed in the cells. The intercellular liquid, or the 

 elaborated fluid which originates in the descending sap, and 

 which has been transmitted to the pith by the medullary 

 rays, is absorbed to the grain of the bud. The diminution 

 which results from absorption begins to dry up the cells, 

 which separate from one another horizontally, Then the slit 

 is formed. We might say that the force of suction, wrought 

 by the bud, took place in the axis of the stalk ; it is, in fact, 

 in this axis that the slit is first formed. These slits are at 



first at great distances from one another. Third period. 



The same facts continuing, results accumulate on results. 

 The circumference alone of the pith still contains any fecule, 

 but this nutritive substance has completely disappeared from 

 the remainder of the pith. The water of vegetation, the 



