CHAP, i The Cell 1 75 



nizable as vacuoles when so swollen. The limiting layer 

 of the vacuole on this hypothesis is the distended wall of 

 the plastid, and is not formed by nor connected with the 

 protoplasmic substance of the cell. 



His views were supported in 1888 by Went, but they did 

 not meet with very general acceptance. The hypothetical 

 tonoplast did not apparently behave after the manner of 

 other plastids of the cell, and did not multiply by fission. 

 Nor was it easy to see how the one large vacuole of the cell 

 could be composed by the fusion of smaller ones, each with its 

 own well-defined wall, without showing the existence of the 

 latter or part of it separate from the general protoplasm. 



The relations between the protoplasm of contiguous cells 

 were almost unknown in 1860. The structure of the sieve 

 tubes had been studied as long ago as 1837 by Hartig, and 

 later by von Mohl in 1855, and Naegeli in 1861, but though 

 their general features had been observed, it was not satis- 

 factorily shown till the publication of Hanstein's researches 

 in 1864 that definite perforations of the walls exist. 

 Nothing further was ascertained till 1882, when Russow 

 and Strasburger independently took up the subject, and 

 both made valuable contributions to our knowledge. In 

 his Bau und Wachstum der Zellhaute, 1882, Strasburger pub- 

 lished a fuller account of their peculiarities, dealing with 

 the formation of the sieve plates and the arrangement of the 

 protoplasmic threads which pass through them. A more 

 exhaustive description of the details of their structure was 

 given by Gardiner and Hill in 1900. 



With the exception of the sieve tubes and certain cell 

 complexes in the red seaweeds, observed in 1883 by Hick, 

 there was no evidence of any intercellular protoplasmic 

 continuity, though it was held by many to be extremely 

 probable ; Heitzmann put forward the theory in 1873, 

 but he failed to produce satisfactory evidence in support 

 of his contention. It was taken up seriously a few years 



