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



tamorphoses which the cell undergoes by the exercise of the 

 vegetable nutrition. In fact the following phases are seen in 

 this pith : — 



First period. — The pith is continuous, full, compact, with- 

 out 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 aliment- 

 ary 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 crystallize by degrees ; the crystals 

 being formed in the cells. 



The intracellular liquid, or the elaborated fluid w^hich ori- 

 ginates in the descending sap, and which has been transmitted 

 to the pith by the medullary rays, is absorbed to the gain of 

 the bud. The diminution which results from this 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 dis- 

 appeared from the remainder of the pith. The water of ve- 

 getation, the elaborated fluid of the sap, is more and more sub- 

 tracted ; 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 sepai'ated one 



