IN VEGETABLE BIOLOGY. 615 



panied by concurrent growth of the nodule to form a ring. 

 Meanwhile the protoplasm between the two pieces of highly 

 refractive substance which form the contour of the upper and 

 under faces of the ring has commenced to differentiate a pit- 

 closing membrane, which either occupies the whole of the space 

 bounded by the ring's contour, with the exception of a single 

 narrow canal at or near the centre, through which the con- 

 necting thread passes, or partially fills up the space, leaving 

 circumferential or scattered pores. Whether the original con- 

 necting filament is always really single from the first is doubtful 

 in such cases as the wide axial cells of Halurus equisetifolius and 

 of the main branches of Ballia callitricha. A typical ring with 

 upper and under highly refractive contour-lines and perforated 

 membrane being formed and increasing in size concurrently with 

 the growth of the cell, the strands of protoplasm passing through 

 the membrane may themselves become surrounded by a small 

 ring, and growing rapidly in calibre just as did the original fila- 

 ment, like it may themselves be divided in passing this secondary 

 ring by the development of a membrane in the latter. I am not 

 sure whether some of the large simple rings before mentioned do 

 not eventually become compound : in Ptilota sericea, for instance, 

 on one occasion a small ring was observed, to which convergence 

 of two strands of protoplasm was plainly made out. This is 

 a matter requiring further elucidation. 



It has frequently been the subject of remark that the parietal 

 protoplasm of the floridean cell is of a denser consistence than 

 that which it surrounds. A glance, for instance, through the 

 plates of Thuret's and Bornet's ' Etudes physiologiques ' will be 

 sufficient to establish this point. Figs. 25 a and 27 of the present 

 memoir may also be quoted in proof of the same. A modification 

 of it consists in the distribution of dense highly refractive proto- 

 plasm in the form of a network upon the wall. This reticulation, 

 originally observed, I believe, by Nageli, seems to be of common 

 occurrence. In most of the types examined by me it is more or 

 less easily to be made out ; but the most beautiful example I 

 have seen is in Polysiphonia nigrescens, from which a drawing 

 of a portion of a siphon-cell's contents will be found at fig. 59. 

 In another modification the protoplasm is arranged in broad 

 lines down the wall of the cell ; good examples of this occur in 

 Ballia callitricha and Halurus equisetifolius. Now the tenour of 

 my observations — inconclusive, it is true, from the want of living 



