17 



or in dilute o-lvcerinp. One can disting'uisli tliree layers 

 in llie cell walls, which are best differentiated in case of 

 condiictina' cells when a lone'itudinal seetiou is stained in 

 hasmatoxyliii and mounted in dilute glycerine. 



The middle lamella is seen to be fairly thin, but can 

 nevertheless be well made out. It is common to all cells. 

 Each cell is surrounded by a wall, which lies immediately 

 inside the middle lamella, and dees apparently not swell 

 up very much in water. Then follows an innermost layer, 

 which is in its turn lined by the protoplasm. This layer 

 shows a very distinct concentric stratification, and is 

 apparently most affected by fresh water. It swells up 

 very much indeed. 



AVith reg-ard to the protoplasm inside the cell wall very 

 little can be said. It consists of the cytoplasm, and con- 

 tains a roundish nucleus, one or more plastids, starch and 

 vacuoles. The cyptoplasm never occupies a ver}^ large 

 space of the cell cavity. The latter is usuallj^ taken up 

 by one or more large vacuoles. The cytoplasm of the 

 larger central cells consists merely of a very fine mem- 

 brane, which lies between the vacuole and the cell Avail. 

 Xo fine partitions formed by cytoplasmic lamelUe can be 

 seen stretching across the vacuoles. 



In the outer collecting, and still more in the assimilating^ 

 cells, the cytoplasm appears as a slightly frothy liquid. 

 Fine lamellae are seen to stretch across the vacuoles. It 

 must, however, be understood that the frothy appearance 

 of the cytoplasm so easily seen in many algro affords no 

 indication as to its ultimate striicture, as is so often 

 supposed. 



It has already been mentioned that the large central 



conducting cells are in communication with one another 



by means of pits. The pit membranes are thin portions 



of the wall which do not swell up in water or glycerine. 



C- 



