94 Rhodora [June 



Under such circumstances the forking and thread-like stems, tipped 

 with rudimentary leaves but otherwise naked, except for the scanty 

 vestiges left behind by the leaves and underleaves, present a very 

 distinctive appearance. The caducous habit is sometimes more 

 marked in the leaves than in the underleaves, and leafless stems 

 with persistent underleaves are occasionally found. There are cases, 

 too, where an axis recovers from the caducous habit and resumes its 

 growth in a normal vegetative manner. 



The caducous leaves are usually smaller than the ordinary leaves 

 described above and may be reduced to a length of 0.2 mm. or even 

 less, the caducous underleaves exhibiting a similar reduction in size. 

 When a caducous leaf gives rise to a new stem the latter grows out 

 directly, by a process of regeneration, from one of the leaf-cells, 

 usually at or near the base. The stem elongates rapidly and produces 

 a long series of minute leaves and underleaves, very similar to those 

 on the flagelliform branches but somewhat firmer in texture and 

 tending to spread more widely. Both leaves and underleaves are 

 shortly bifid and essentially alike, the stem thus representing an 

 almost radial structure and showing but little indication of the dis- 

 tinct dorsiventrality found in the normal leafy stems. The later 

 stages in the development of these new stems have not been observed. 



When typical plants of B. tricrenata and B. denudata are compared 

 the differences between them are striking and have been brought out 

 to a certain extent in the preceding account. In B. tricrenata, for 

 example, the plants are more or less pigmented with brown, the 

 stems tend to be suberect, the branches diverge at a narrow angle, 

 and the persistent leaves are strongly convex; in B. denudata, on the 

 contrary, the plants show no distinct brown pigmentation, the stems 

 tend to be prostrate, the branches diverge at a wide angle, and the 

 leaves are flat or only slightly convex and often caducous. When 

 B. denudata is compared with " Pleurosehisma tricrenatum var. 

 implexum" a closer approach is apparent, but in this latter plant the 

 pigmentation is still present in a greater or less degree, the caducous 

 habit is less marked and the caducous leaves themselves are less 

 highly differentiated. It will be noted that " Mastigobryum ambiguum 

 (in part)" is cited as one of the synonyms of B. tricrenata. This 

 species was based on two specimens, as follows : " prov. Massachusetts 

 (Asa Gray)" and "ad litora boreali-occidentalia (Hb. Hk.)." The 

 first specimen has not been seen by the writer but there is little 



