J lone: PETALONEMA ALATUM IN MINNESOTA. 49 



longitudinal striae, and the mouths of the younger sheaths, pro- 

 jecting slightly beyond those of the older, form the crenatures 



of the margin.'' 



I find the central cylindrical column containing the trichome 

 to be dense and often very thick near the heterocyst {Fig. 4), 

 while near the apex of the filament it generally becomes thin 

 and often scarcely traceable {Fig. j). Thus it would seem to 

 be a second sheath within the larger outer one, or it may be 

 merely a very dense interior folding of the sheath proper, inten- 

 sified near the heterocysts because of the greater rigidity of 

 that portion. The internal striations of the sheath have a 

 beautiful golden or dusky brown feathery appearance. Those 

 farthest within being brown or golden present all shades of 

 brown and yellow as they approach the periphery, where the 

 sheath becomes colorless and transparent. A quite common 

 condition is illustrated in Fig. 5, in which a dense old sheath, 

 contracted for a limited distance, after branching expands into 

 the usual form. 



Trichome. — The trichome is normally of an olive-green 

 color, cylindrical or somewhat moniliform, separated into dis- 

 tinct pseudocysts or apparently continuous. The apex is often 

 gradually constricted with the tip enlarged. Very often this tip 

 is rose-colored. 



Pseudocysts. — The pseudocysts are exceedingly variable in 

 size and shape, this depending upon their age. The younger 

 active pseudocysts are globose in shape, 9-15 mic. wide. 

 When ready to divide they lengthen until the}' are twice as 

 long as wide (Fig. 7). The contents are coarsely granulate. 

 Often a single large granule is seen in each pseudocyst (Fig. 

 j). The older pseudocysts are often rectangular in outline 

 (Fig. 4, a). These are sometimes 20 mic. long and 6 mic. 

 wide. They are more finely granular and densely packed. 

 The apical pseudocyst and often three or four below are 

 coarsely granulate and are of a deep pink or red color. 



Heterocysts. — The heterocysts are interstitial and sometimes 

 occur at the base of a branch (Fig. 5). They are solitary. In 

 shape they are somewhat globose or oblong. In stained mate- 

 rial the watery contents take a beautiful color. In size they are 

 slightly larger than a normal pseudocyst. 



