31 



commencement of a proper vessel, which probably in this case 

 endeavours to produce a new anastomosis between two trunks 

 having a longitudinal course, and the horizontal cellular walls 

 still appearing in the form of delicate intermediate parietes, as 

 those parts which in the further progress of development are 

 gradually obliterated. Subsequently he calls these statements 

 and suppositions a history of the development of the proper 

 vessels, and thinks that the formation of the spiral tubes is 

 exactly similar. My observations on this subject are quite op- 

 posed to the suppositions of M. Unger. In the earlier stage, 

 the walls of these vessels are not yet perceptible, and the milky 

 sap is found as it were between the cells in uninterrupted cur- 

 rents : at a later stage, the sides of these vessels gradually in- 

 crease in thickness and become more and more distinct from 

 the adjacent cellular walls ; nay, in some parts of plants, as for 

 instance, in the root, &c., they are furnished by the adjacent 

 cells with more or less strong constrictions, so that even forms 

 similar to those designated by C. H. Schultz as articulated 

 vessels arise. 



ec Since these vessels," observed M. Unger, " are developed 

 from cells, the peculiarities in respect to the structure of 

 their parietes may also necessarily be applied to these vessels. 

 As we may regard the parietes as formed of two more or less 

 adherent lamellae (or, according to my opinion, as originally a 

 simple membrane gradually dividing into two lamellae), we can 

 scarcely deny to the proper vessels one constituent part of its 

 external boundary peculiar to them alone," &c. 



I have in my memoir on the Secreting Organs of Plants 

 (Berlin, 1837, 4to.) and in my Vegetable Physiology (ii. p. 

 370 428), expressed myself in more distinct terms on this 

 subject ; I think that botanists must admit the peculiar pa- 

 rietes of the milk sap vessels, because they really exist in na- 

 ture, and are evident to every one who searches for them ; if, 

 however, M. Unger has not yet observed the circulation of the 

 sap in the vessels of Ficus bengalensis, he will probably notice 

 it hereafter, for it really exists. 



M. Mandl* has published some results of his observations 

 on the milky sap of plants, which highly deserve the attention 

 * L'Institut de 1837, p. 127. 



