346 



Dr. D. Ellis. 



[May 17 ; 



usually of long colourless threads of 1-3 fi in width. When undisturbed, the 

 length may reach up to 3 mm. Each thread consists of a single row of rods 

 enclosed in a delicate sheath. The thread may be free, or attached to blades 

 of grass or other similar objects. When free, the thread may be floating or 

 it may assume motility. Its general microscopic appearance is shown in fig. 1 

 (Plate 8). The rod-shaped cells vary in length, but the average is 3-5 times 

 the width. When the thread is attached the cells at the free end are, on 

 the whole, longer than those at the other end. In some threads the cells 

 may be some distance apart (fig. 2). The falsely dichotomous appearance 

 of this species (fig. 3) is seen only when the stream is moving very 

 slowly or is at a standstill. False dichotomy is due to the growth into 

 threads, of cells which have slipped out sideways. In my own cultures 

 its formation was quite exceptional. 



The Sheath. — The sheath forms a hollow tube, closed at both ends when the 

 thread is young (fig. 4). In the older condition, if attached, the thread opens 

 at the apex, when the enclosed cells escape into the surrounding water 

 (figs. 2 and 18). It is generally assumed that the sheath is a delicate 

 but firm textured envelope in the form of a hollow cylinder, and that this 

 condition is maintained throughout the life of the plant. All my 

 observations show, however, that the sheath is a clinging mucilaginous 

 envelope during the early stages of growth and only later assumes a firm 

 consistency. It is developed by a mucilaginous secretion of the walls of the 

 young cells. In very young threads, composed of two or three cells only, 

 there is no trace of a sheath. This structure can best be studied by means of 

 iodine preparations, for the cells become deeply stained, while the sheath, 

 though remaining uncoloured, becomes more opaque; further, the cells 

 contract somewhat, thus making the sheath a more conspicuous object (fig. 5). 

 In normal healthy threads, before the splitting of the sheath at the apex, 

 there are transverse bars of the same material as the sheath between each cell. 

 These can best be demonstrated by leaving freshly collected threads in ether 

 for two or three days in order to cause the cells to contract (fig. 6). The 

 contents of the cells will have then become disorganised and the cells 

 themselves noticeably smaller. The same result can be produced by iodine if 

 material at the right stage be selected (fig. 7). These bars are also often seen 

 in old threads that have become partially disorganised (figs. 8 and 9) ; in such 

 cases the cells decompose faster than the sheath. Previous writers make no 

 mention of these transverse bars and the only indication that I can find of 

 their existence in the works of previous writers is in fig. 17 of Bxisgen's (2) 

 publication. On p. 151 he says, "Von anderen Entwickelungsstanden der 

 Cladothrix traten in meinen Culturen nur noch eigentumlich gestaltete Fatten 



