Growth of Cells in Polysiphonia. By George Massee. 199 



To understand clearly the structure of the growing point, it is 

 necessary to examine specimens from which the cell-walls have 

 been removed: this can be accomplished by soaking for several 

 hours in a solution of nitro-picric acid. If this material is stained 

 and examined under a high power, it will be seen that the proto- 

 plasm of each cell is in perfect continuity with the next above and 

 below, being connected by a narrow neck of protoplasm. In 

 addition to this central string, which is comparatively thick and 

 strong, much finer threads may sometimes be seen springing from 

 one of the masses of protoplasm just within its margin, and joining 

 on to the next mass in the same position. 



These marginal strings were for some time a source of much 

 perplexity, as owing to their extreme tenuity they rarely survived, 

 unbroken, the treatment necessary for the removal of the cell-walls, 

 and, if broken, the protoplasm contracts so much, that not a trace 

 of the torn ends remain. (PI. YI. fig. 1.) 



If a small amount of pressure, combined with a rotatory move- 

 ment, be applied to the thin glass cover protecting the specimen 

 under examination, the segments of some of the growing points will 

 be separated from each other. Examined in detail, these segments 

 in Polysiphonia urceolata present the following appearance. The 

 first below the apical cell resembles a thin circular disk with an 

 unbroken margin ; the second working backwards from the apex is 

 slightly thicker than the preceding, and the margin has four 

 notches at equal distances ; these notches, in the third segment, 

 reach about half-way from the centre to the circumference of the 

 disk, which thus presents the appearance of a Maltese cross ; in 

 the segments further back, the four lobes are more or less quadrate 

 in form, and each is joined to the central mass of the segment by 

 a narrow neck of protoplasm (fig. 2). The central mass of the 

 segment developes into the axial cell of one joint of the stem, and 

 the four outgrowths form the four cortical cells. The slender 

 threads, alluded to as springing from near the margin of the disk, 

 are in P. urceolata four in number, and unite the superposed 

 cortical cells of adjoining segments. The cortical cells are thus 

 connected with each other by vertical protoplasmic threads, each 

 one again communicating with the axial cell by a horizontal thread, 

 while the axial cells, as already shown, are connected by vertical 

 threads (fig. 3). This mode of formation of the cortical cells, 

 as also their connection with each other and with the axial cell, is 

 the same in all the species of Polysiphonia that I have had an 

 opportunity of examining. 



The tetragonidia originate in precisely the same way as the cortical 

 cells ; in Polysiphonia fastigiata they occupy a space equal to 

 two of the latter, from which they are readily distinguished, even in 

 the earliest stages of development, by their more spherical form, and 

 by the presence of two prominences on the peripheral margin, which 



