SPIROGYRA 125 



in thickness, and each cell may have an additional thin pectose 

 wall and an inner cellulose wall next the protoplast. The trans- 

 verse or end walls of the cells at maturity are of 4 types: plane, 

 replicate, semireplicate, and colligate. These are probably funda- 

 mental cell differences. The first three types have long been used 

 to separate three primary groups within the genus. 



Following cell division the primary wall, or middle lamella of 

 the partition wall, is composed of a pectic compound and has a 

 plane surface. The secondary wall of cellulose also is plane in 

 208 species. In 65 species the cellulose layer develops a circular 

 cuplike infold. (PI. XXXV, Figs. 5-20.) Two other species have 

 only a semicircular infold. (PI. XXXV, Figs. 1-4.) In the adja- 

 cent cell the infold is opposite the plane half so that when these 

 partition walls are seen from the side, the two folds resemble a 

 partly open transom. 



Not infrequently, in the division just preceding the formation 

 of gametangia, the walls are plane even in the replicate species. 

 There is i species (S. colligata) in which the middle lamella con- 

 tinues to grow in thickness and spread out over the ends of the 

 cylindric wall. This process results in a collared diaphragm be- 

 tween successive cells. (PI. XXXI, Figs. 11-13.) Isolated exam- 

 ples of collared diaphragms may be found in other species. 



When cells with plane walls separate, the free ends become 

 rounded. Free end walls of replicate species have a narrow central 

 bulge with a distinct shoulder. The free ends of semireplicate 

 cells have an asymmetric pointed bulge on the half where the 

 infold occurred. As cells age, the transverse walls may become 

 thickened and distorted through pectinization. 



The replicate walls are curious structures. Since they first were 

 seen, botanists have exercised their ingenuity to devise some im- 

 portant use or advantage for them. Frequently they have been 

 held up as special adaptations for fragmentation. Yet all these 

 botanists know that plane-walled filaments fragment without dif- 

 ficulty. When fragmentation occurs throughout a filament, the 

 protoplasts are on their way to disintegration, not to multiplication. 



The nucleus of a vegetative cell is located near its center, 

 enclosed in cytoplasm supported by cytoplasmic strands, the outer 

 ends of which usually adjoin a pyrenoid. 



The simplest method of propagation and survival over a dor- 

 mant period is by akinetes. These may be cylindric thick-walled 



