i6 ZYGNEMATACEAE 



specimens with mature spore walls can be named with any degree 

 of satisfaction. Not infrequently the ornamentation can be deter- 

 mined only by crushing and separating the several layers of the 

 spore wall. In the matter of dimensions, the student should 

 remember that every species is a complex of clones with cells of 

 slightly different sizes and proportions. These may be in part 

 smaller or larger than the dimensions given in the key. The 

 dimensions given in the descriptions of species are either those 

 of the original collection, or the dimensions as they may have 

 been modified by the study of subsequent collections. How great 

 a departure from the original is necessary to warrant the segre- 

 gation of a new species cannot be determined by any simple rule. 

 Study and restudy of many collections will show that some species 

 are only narrowly variable, while others vary within wide limits. 

 These remarks are equally applicable to the species of other genera 

 of the Zygnemataceae. 



In the descriptions of species the characteristics of the outer 

 spore wall are usually omitted since there are only a few species 

 in which they are not thin, smooth, colorless, and transparent. 

 The median spore wall may be variously colored and ornamented. 

 I have tried to distinguish among punctate, scrobiculate, and pit- 

 ted according to the size of the pits, and in many instances have 

 been able to give the diameters of the pits and the distances be- 

 tween them in microns. Errors sometimes occur in descriptions 

 and figures where contraction ridges and irregular folds are mis- 

 taken for ornamentation. These may be due to contraction of the 

 spore contents either by drying or by plasmolysis, and may be 

 recognized readily by applying a dilute solution of potassium 

 hydroxide. Real structural patterns are enlarged and made more 

 distinct by this treatment, while contraction ridges disappear. In 

 studying dried specimens it is well first to wet them with water 

 on the slide and then to apply a drop of lactic acid, and after that 

 to heat the slide until the acid begins to boil. This treatment will 

 remove calcium carbonate and clarify both the walls and cell 

 contents of most species. Some of the older species were described 

 as having smooth median spore walls, although recent study of 

 the type specimens has shown them to be punctate. This circum- 

 stance is probably explained by the poor resolving power of the 

 microscope lenses of the last century as compared with those 

 available today. 



