198 Transaotions of the Society. 



YII. — On the Formation and Growth of Cells in the Genus 

 Polysiphonia. 



By Geobge Massee, F.E.M.S. 



( Read 12th March, 1884.) 

 Plate VI. 



If the growing point of Polysiphonia is examined under a low 

 power, a plano-convex apical cell containing a nucleus is seen, the 

 convex side being uppermost ; below this — depending on the rate 

 of segmentation of the apical cell — ^from two to four thin disk-like 

 segments are superposed ; further away from the growing point, as 

 the segments increase in breadth, each is seen to be surrounded by 

 a row of cortical cells, the so-called " siphons," these latter appearing 

 to be absent from the youngest segments lying immediately below 

 the apical cell. 



EXPLANATION OF PLATE VI. 



Fig. 1. — Portions from growing point of Polysiplwnia urceolata, showing the 

 protoplasmic threads connecting superposed segments. The cell-walls have been 

 removed, x 750 diam. 



Fig. 2. — Transverse section through the stem of P. urceolata at the point 

 where the protoplasm of the cortical cells is continuous with the protoplasm of 

 the axial cell, x 500 diam. 



Fig. 3. — Diagrammatic representation of the protoplasmic portion of the 

 growing point of P. urceolata. 



Fig. 4. — Transverse section through the stem of P. fastigiata at the point 

 where the axial cell is connected with the cortical cells ; a, tetragonidium ; b, 

 two cortical cells produced from the tetragonidium by gemmation. The cell- 

 walls have been removed, x 500 diam. 



Fig. 5. — Vertical section through the growing point of P. urceolata, showing 

 the ingrowth, by degrees, of the transverse walls. The protoplasm has been 

 removed, x 750 diam. 



Fig. 6. — Perforated plates of cellulose removed from the openings left in the 

 transverse septa of the axial row of cells in P. fastigiata. x 1000 diam. 



Fio-. 7. — Portion of axial cell of P. fastigiata, showing the perforation through 

 the transverse wall at a, from which a perforated disk of cellulose similar to 

 fig. 6 has been removed .; b, minute holes in the cell-wall, through which proto- 

 pfasmic threads pass from the axial to the cortical cells, as shown in figs. 

 8 and 9. 



Fig. 8. — Transverse section through an axial cell of P. fastigiata at the point 

 where the protoplasm is giving off rays, a, to the cortical cells. The cell- wall is 

 thickened and stratified, x 750. 



Fig. 9. — Vertical section of axial cell of P. fastigiata, showing stratified cell- 

 wall and protoplasm givtug off rays, a, to cortical cells, x 750. 



Fif. 10. — Vertical section tlirough a branch of P. urceolata a short distance 

 from the apex; the cells have grown considerably in a radial direction, but very 

 little vertically. X 750. 



Fig. 11. — Vertical section through a branch of P. urceolata at some distance 

 from the apex, showing vertical growth of cells, x 750. 



Fig. 12. — Axial cells from P. fastigiata close behind the apical cell, showing 

 the origin of cortical cells by gemmation, x 1000. 



Fig. 13. — Young tetragonidia from P. fastigiata. x 1000. 



Fig. 14. — Growing points of P. fastigiata, showing method of segmentation of 

 apical cell for formation of a dichotomy. 



