Winslow: Double Sori in Athyrium 91 



At first thought a hunt for double sori on A. angusti- 

 folium would seem like a hopeless quest, but such have 

 been seen and reported by one author at least, Miss 

 Slosson.* In the main, the long, parallel sori are laid in 

 regular order on the upper sides of the upper branches 

 of the once forked veins. As there are no lobes or angles, 

 there is no opportunity for double sori. But where the 

 pinnae dwindle to insignificance toward the top of the 

 frond and pass into the graceful accuminate tip, the sori 

 change from the outer to the inner and upper side of the 

 veins. That is — where there are no pinnae the sorus 

 goes to the side nearest the tip of the frond. At the 

 point where this change occurs, if anywhere, we should 

 find double sori. The plant seems rather averse to this 

 arrangement and the first two or three fronds examined 

 had two or three of the last tiny pinnae entirely sterile, 

 although there were sori above and below. But about 

 the fourth frond showed one solitary pair on the very 

 last lobe that could be called a pinna. (Figure 2.) 



This somewhat superficial treatment of the subject 

 seems to emphasize the similarities of these three species 

 rather than their differences. The conclusion seems to 

 be that unilateral sori rightly situated relatively to the 

 lobes and branches of the midvein are likely to appear 

 diplazioid in any species, and that double sori are of very 

 little diagnostic value. Whether all curving of sori in 

 A. filiz-femina, for instance, can be accounted for as a 

 weak manifestation of the doubling tendency is a ques- 

 tion for further consideration. In general, forms with 

 much curved indusia also have strongly lobed and incised 

 margins, and where the sorus is completely horse-shoe 

 shaped, as in the cyclosorum forms, each sorus is in the 

 position where a double sorus might be expected. 



Some authors apparently regard the double sorus as 



♦ •* 



How Ferns Grow," Slo— on. 



