CONFERVOID FILAMENTS OF MOSSES. 573 



cellulose. It bears a considerable resemblance to the beterocysts of Nostoc (Collema- 

 gonidium) and the so-called Nostocbacese, considered by Braun * as tbe first appearance 

 of vegetative permanent cells, — although he considers these to be formed by a cell divi- 

 ding into two unequal halves, one of which remains permanent, and the other continues 

 to segment. 



It has abeady been remarked that these filaments will soon cover a large space of the 

 ground in their neighbourhood with their branches, if they are permitted to grow unre- 

 strained ; but after a time, particularly in damp, shady situations, the numerous branches 

 begin to crowd upon each other so as to form a compact mass of filaments (PI. LVIII. 

 fig. 16) ; sometimes these intimately interlace, but more often they run upwards, parallel 

 to one another (PL LVIII. fig. 16, PL LVII. fig. 7). Very commonly these branches 

 are of nearly equal length, so that the surface on which they grow appears clothed with 

 velvet. Sometimes, when they press closely upon one another laterally, they become 

 hexagonal, and the cells of the lower portion, deprived of their due quantity of light, 

 become nearly, and occasionally entirely, colourless. 



The foregoing description applies to confervoid filaments grown under circumstances 

 favouring active vegetation, namely, under warmth and moisture, such as in the shade 

 of walls, banks, and sides of ditches. The same can also be very readily produced arti- 

 ficially, by keeping them under a glass shade in a warm room, without direct sunlight. 



But drought and cold produce very marked changes in their outward form and appear- 

 ances, which I shall endeavour to describe. 



Under di'ought, more particularly during summer, the growth of the cells is much 

 checked, and at the same time the ceU-wall becomes much thicker. The cell-contents 

 still growing, the chlorophyll-utricles crowd the interior, and press upon each other 

 (PL LVII. fig. 11). Sometimes the ceU-waU is itself dyed with chlorophyll, and some- 

 what undefined from the contents, like the segmenting gonidia of Collema — a condition 

 not very nn common in the segmenting cell generally. This tendency to the blending of 

 the contents with the ceU-waU I have endeavom'ed to show at PL LVII. fig. 11 b. It is 

 very curious to observe the effects produced by alternations of weather upon these fila- 

 ments, illustrations of which may be procured artificially. Some of the cells are long, 

 with few granules ; some are short, and the contents closely packed. Some have begun 

 to branch, and become suddenly arrested in the process, the bulging part becoming 

 covered with a dense ceU-waU, precluding further growth in that direction. Some of 

 these branches, encouraged by warmth and moisture, have rapidly grown out into narrow 

 elongated cells, so as to look something like roots (PL LVII. fig. 11 b). As any part of 

 the cell is capable of bulging, and as any one of the cells can do so independently of the 

 other, and as the rate of growth of one cell is not limited by that of its neighbours, it can 

 easily be imagined how variable and tortuous may be the forms which these confervoid 

 filaments assume under the varying influences of our climate. Perhaps it may be best 

 observed in those filaments which inhabit the bark of trees or dry walls, where the 

 eflPect of the alternations of the seasons is more marked. 



A very common effect which drought produces or increases is the development of a 



* Rejuvenescence in Nature, Ray Soc. 1853, p. 146. 



