Mucus-substance of the Cell in the Characese. 17 



aggregations are most prominent midway between the lines of 

 repose, so the greatest impetus is given to the axial fluid and 

 its suspended bodies in this part, while at the sides, that is, 

 close to the lines of repose, it is remarkably slow. This was 

 the reason why Slack* thus designated them; but he was 

 wrong when he conceived that the dark defined surface of the 

 mucus-layer was a membrane which was fixed to these lines 

 throughout their course, and therefore divided the ascending 

 from the descending current, inasmuch as the opposite currents, 

 which are closely approximated at these parts of the internode, 

 frequently take from each other the particles which are lingering 

 along their direction, and thus whirl them backwards and for- 

 wards, or up and down, according to the position of the inter- 

 node, until, by chance, they get into a more powerful part of 

 the stream, and are then carried round the internode with the 

 rest, — which would not be the case if a partition existed along 

 the lines of repose. 



Easy, however, as it is to describe the way in which the axial 

 fluid is circulated, it is not so easy to describe the property by 

 which the mucus-layer travels round the internode. 



That Amici and Dutrochet should have ever considered the 

 green disks instrumental in any way in effecting this in the 

 internode of the stem, shows that neither of them recollected at 

 this time, that the mucus-layer moves round the internode of 

 the roots of the Characese in just the same manner, where there 

 are no green disks present ; while the total absence, or rare 

 occurrence, of gonidial cells in the internodes of the roots of 

 Characese, we now know to be an additional reason for inferring 

 the absence also of the green disks there, even if they were 

 present without their green colour to make them more visible. 



That the green disk is occasionally seen to move per se, and 

 to rotate actively in situ, has been observed by others as well as 

 myselff; and the observations of Donne, confirmed byDutrochetJ, 

 prove, that under certain circumstances, viz. when in a globule 

 of the mucus-layer, they will rotate also out of the inter- 

 node; but these are exceptions to the general rule. Again, 

 out of many scores of observations, I have seen only two in- 

 stances in which the green disk (in an old internode, where 

 most of the contents had passed into gonidial cells) presented 

 each a cilium ; also, on one other occasion, three green disks in 

 situ, in an internode where the circulation was going on rapidly, 

 presented each a short thick cilium in active motion ; but these, 



* Trans. Soc. of Arts, vol. xlix. pt. 1, 1833. 

 t Varley, Trans. Soc. of Arts, vol. xUx. pt. 2, 1833. 

 X Ann. des Sc. Nat. 2<^ Ser. t. x. 1838. 

 Ann, is Mag, N. Hist. Ser. 2. To/.xvi. 2 



