348 



Conjugate 



8. neglecta (Hass.) Ktitz., their margins are quite smooth and there is a 

 regular axile series of pyrenoids ; in others, as in 8. nitida (Dillw.) Link or 



8. porticalis (Vauch.) Cleve, 

 the chloroplasts are very broad, 

 with serrated margins and 

 scattered pyrenoids. Between 

 these two extremes there is 

 every gradation, and the char- 

 acter of the chloroplasts always 

 remains constant for any par- 

 ticular species, even though 

 the number of them may vary 

 in different cells of the same 

 filament. It has been ascer- 

 tained by Kolkwitz that the 

 chloroplasts grow in length in 

 the direction of the coils by 

 both apical and intercalary 

 growth ; and therefore as this 

 is obliquely to the surface of 

 the cell-wall, there is a gliding 

 motion of the spiral bands 

 through the lining layer of cytoplasm. The chloroplasts of any one cell in 

 most cases continue along the exact spiral line of those in adjacent cells, 

 a fact which gives the thicker species with several chloroplasts the appear- 

 ance of having continuous spirals, more especially when viewed under a low 

 magnification. 



The nucleus, which in some species is transversely ellipsoid or complanate 

 and in others globose, is in all cases suspended in the median part of the cell, 

 embedded in a small amount of cytoplasm from which radiate numerous 

 protoplasmic strands to the lining layer (fig. 217 A). Many of these strands 

 reach the lining layer exactly opposite a pyrenoid, and, since the starch-grains 

 are laid down around the pyrenoids, this fact has been brought forward as a 

 reason for supposing the nucleus to have a direct influence on starch- 

 formation. In some species, such as Spirogyra majuscula Ktitz. and 

 8p. pellucida (Hass.) Ktitz., the nucleus is clearly visible in the living cells. 

 Gerassimoff ('97 ; '00) has found that the cells are sometimes binucleated or 

 they may contain a ' compound nucleus.' 



In cases of this kind the abnormal nuclear structure is owing to adverse conditions 

 arising during nuclear division. If sudden cooling occurs at the very commence- 

 ment of nuclear division it may result in the production of two cells, one with an 

 extraordinarily large nucleus and one without any nucleus; if the cooling takes place 



Fig. 217. A, single cell of Spirogyra crassa Kiitz., x 350, 

 showing the spiral chloroplasts with their pyrenoids 

 and the centrally suspended nucleus with its 

 nucleolus. B, the replicate extremities of adjacent 

 cells of Sp. tenuissima (Hass.) Kiitz., x 1000. G, the 

 same with the infoldings everted. 



