74 Prof. C. Morren on the discoid Piths of Plants, 



It is highly probable that this structure is much more com- 

 mon than is generally supposed, since by counting the Begonicd 

 among those plants which present it to us, as we have ob- 

 served, we find that the discoid piths exist in the families of 

 the Santalacece, Juglandece^ Phytolaccece, Jasminece, and Bego- 

 niacece, which have no similarity with each other. This in- 

 duces me to suppose that further inquiries will prove the ex- 

 istence of this form in many more plants. I should however 

 remark that species of the same genus may in this respect 

 completely differ. Thus Begonia argyrostigma presents a dis- 

 coid pith, whilst Begonia undulata, B, semperflorens, B, pa- 

 pulosa, B, dichotoma, have a continuous pith*. Thus the 

 Phytolacca decandra is furnished vrith a discoid pith, whilst 

 that of the Phytolacca dioica, Ph, stricta, is continuous, &c. 

 Some similar examples might also be taken from the genus 

 Jasminum, the genus Nyssa, &c. This structure therefore de- 

 pends upon some circumstance wholly specific, which neither 

 extends to the genus nor to the family, and which consequently 

 could never become of any importance in methodical distri- 

 bution*. 



Observation however has taught us that the physiology of 

 the pith of dicotyledons would receive useful and remarkable 

 improvements from the careful study of the formation of 

 these medullary discs, for it is known that to few parts have 

 been assigned so many different uses as to the pith. It was 

 considered by some as the nervous system of the plant, even 

 as the brain of this system ; by others, and that not long ago, 

 as the apparatus of the ascension of the stem ; for, said they, 

 if the brain is placed in the head, the organ which is placed 

 in front in the progression of animals, the pith is placed in the 

 stalk, the organ which progresses upwardly in a plant. Then 



*■ In vegetable physiology great attention has been paid to the secretions 

 of the pith and consequently to its colour : it is correctly said to be green at 

 its origin and ivhite or brown at a later period and when it is dead. But 

 the Begonia papulosa exhibits the phaenomenon of a beautiful retfpith when 

 it arrives at maturity. The red is as bright as that of the beet-root, and 

 this colour is caused by a red liquor without globules, which fills the prismen- 

 chymatous cells of this pith, so that there is no difference between the kind 

 of coloration of this part, which occupies the interior axis of the plant, and 

 that of the most peripherical organs, such for instance as the derm, the pe- 

 tals. This fact corroborates the opinion that in this phaenomenon the work 

 is all cellular, that is to say, it resides in the cell itself and proceeds from it. 



