244 PLAGIOTHECIUM MULLEPJANUM IN BRITAIN. 



the latter dimensions. (Limpriclit, however, gives the measurement 

 as 5-6 /x.) 



But it is in the appearance and structure of the stem that the 

 most sahent characters are found by which P. Mullerianwn may be 

 distinguished with ease from the alhed species. If a moderately 

 robust branch of P. Mi'dlcrianum be laid side by side with branches 

 of the two other species, and viewed under the microscope or with 

 a lens, it is at once noticeable that the stem of the former is much 

 stouter than in either of the two latter. The actual thickness of the 

 normal stem in P. pulchellum var. nitididum may be put at about 

 80-120 /x, attaining a maximum of 150 /x, that of P. Borrerianum 

 being about the same. In P. Mulleriaimm an ordinarily robust stem 

 ranges in thickness from 150 to 200 /x, frequently exceeding this, 

 while it occasionally attains a thickness of 2G0 /x. (In slender forms 

 of the plant, however, as in the flagelliform branches often produced, 

 the thickness is much less.) 



This marked difference in the thickness of the stem affords a 

 character by which one accustomed to the plant could distinguish 

 it, when typical, from the other two, even with a lens; but there is 

 a still more important and constant character by which under the 

 microscope it may be at once recognized even in its most attenuated 

 forms : this is the structure of the stem tissue. 



A transverse section of the stem of P. imlchellum shows a faint 

 central strand, which is wanting in P. Mullerianum, and the cortical 

 cells in the former species as well as in P. Borrerianum. are narrow, 

 with the walls slightly thickened, while in P. MuUerianum they are 

 large, loose, and thin- walled. Limpricht italicizes the description 

 of these "sphagnum-like" cortical cells as a distinguishing cha- 

 racter, but makes no further reference to them ; Sullivant, too, 

 gives a figure of a transverse section of the stem showing them, but 

 makes no mention of them whatever in his description; and neither 

 author points out the fact that the character is easily observed 

 without making a section of the stem, and is one by which P. Mill- 

 lerianum may be at once separated from either of the two species 

 with which it is likely to be confused. 



If a stem of P. pidcheUmn or P. Borrerianum, partially denuded 

 of leaves, be placed under the microscope, the cortical cells are seen 

 to be narrow and obscure, measuring abouc 6-8 /x in width, thus 

 being practically about the same width as the lower cells of the leaf, 

 narrower indeed than the extreme basal cells at the line of insertion. 

 In P. Mullerianum, on the other hand, the cortical cells range from 

 about 14 /x in width to as much as 28 /x, averaging about 20 /x; thus 

 measuring about four times the width of the lower leaf-cells, and 

 two or three times the width of even the widest extreme basal cells. 

 A reference to the plate (figs. 4, 5) will show more clearly than any 

 description the ready character which this affords to distinguish 

 P. Mullerianum. 



In its fruiting characters P. Milllerianum appears to differ but 

 little from P. jnilcJiellum., though Limpricht points out one or two 

 minor points of difference. 



