126 ZYGNEMATACEAE 



cells, or the thickening may be unequally distributed and modify 

 the usual cell form. (PI. I, Fig. i.) Aplanospores are formed by 

 the contraction of the contents of vegetative cells and the growth 

 of a new and distinctive spore wall. 



The "spore wall" is really a complex of three to five walls, one 

 or more of which may be variously sculptured or ornamented. 

 The outer wall is of cellulose and is usually colorless and trans- 

 parent. Sometimes the outer wall is made up of two colorless 

 layers, either of which may be thick and sculptured. The median 

 spore wall at maturity is distinguished by its yellow to chestnut 

 brown color resulting from chitinous deposits in or on the cellu- 

 lose. This is the spore wall most frequently sculptured and orna- 

 mented. In some species the median wall also consists of two 

 distinct chitinous layers. The inner spore wall is a thin cellulose 

 layer lining the median wall and seldom seen, except when the 

 spores are crushed. The median and the outer walls have a suture, 

 more or less distinct, encircling the spore. At the time of germi- 

 nation these walls split along this line, while the inner wall 

 enlarges with the protoplast and becomes the wall of the one 

 celled sporeling. The walls of zygospores are similar to those of 

 aplanospores. 



The forms of Spirogyra spores are of three general types: 

 ellipsoid (like an American football), ovoid (like a watermelon), 

 and lenticular (a compressed spheroid). These forms may be 

 slightly modified by elongation or compression but the curvatures 

 of the polar ends remain the same. Usually there is no difficulty 

 in recognizing the spore form of the 155 species with ellipsoid 

 spores, and the 77 species with ovoid spores. There are 13 species, 

 however, in which there is greater variation, and the form may 

 be on either side of the border line between ovoid and ellipsoid. 

 These are usually cared for in the keys by listing under both types. 

 Compressed-ellipsoid, and compressed-ovoid spores are rare but 

 do occur. The deviation of "lenticular" spores (30 species) from 

 the form of a sphere may be as slight as an orange, or as great as 

 a pocket watch. Obviously such spores are nearly circular in face 

 view, and ovoid in side view. 



Conjugating tubes are usually formed by both gametangia 

 (228 species) but in 31 species they are outgrowths of the male 

 gametangia. In a few species in which the receptive gametangia 

 become greatly distended the part of the tubes formed by them 



