SPIROGYRA 127 



may be widened and nearly disappear, leaving the tubes "appar- 

 ently formed by the male gametangia." 



Another matter that must be determined early in the use of 

 the key concerns the growth of the female, or receptive, gam- 

 etangia during conjugation. Do they remain cylindric, or become 

 slightly enlarged, or greatly distended or inflated ? The two latter 

 conditions we judge by the enlargement relative to the thickness 

 of the spore. If the spore just fits the distended part of the gam- 

 etangium it is enlarged. If gametangial walls are much more 

 distended than the diameter of the spore, the gametangium is 

 inflated. The inflation may be only on the conjugating, or inner, 

 side, or it may be only on the opposite, or outer, side. In many 

 species the inflation is on both sides. 



The gametangial walls are not distended by the spore walls. 

 Contrariwise the spore dimensions and forms are often limited by 

 the gametangial walls. One can find many examples to prove this 

 where spores formed in small gametangia are variously deformed 

 by the rigidity of the gametangial wall. 



Another interesting fact is that there is no proportional relation 

 between the combined volumes of the gametangia and the vol- 

 umes of the spores in different species. Spore sizes in the same 

 species, however, are usually larger or smaller depending upon 

 whether the volumes of the gametangia are larger or smaller than 

 the average. 



It is noteworthy that small spores in large gametangia are not 

 free to move about as the cell is turned. The spores in all the 

 Zygnemataceae are more or less fixed in position by an extremely 

 dilute pectic gel that fills the entire cavity of the "empty" gam- 

 etangia and sporangia. If these spaces were filled with water alone 

 the position of the spores could be changed readily. 



The last essential feature of the description of a Spirogyra is 

 the number of spore walls, the color, and surface markings of 

 each. The kinds of ornamentation are best shown by illustrations. 

 Not all drawings are equally satisfactory, but some of the most 

 difficult have been sketched with remarkable accuracy. 



Spore wall features should be seen through an oil immersion 

 lens, so that one can be sure to distinguish between the shadows 

 of spore contents and actual wall structures. In drawing spores 

 the contents should be omitted, since they are of no taxonomic 

 value, while the wall characteristics have great importance. 



