580 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



have been produced l>y binary subdivision from a single parent- 

 cell remain adherent one to another in linear series, so as to form a 

 filament (fig. 440; Plate IX, fig. 3). They are distinguished by two 

 peculiar features, one of these being the semblance of a division 

 of each cell into two symmetrical halves by a ' sutural line,' which 

 is sometimes so decided as to have led to the belief that the cell 

 is really double (Plate VIII, figs. 2, 6), though in other cases 

 it is merely indicated by a slight notch ; the other feature is the 

 frequency of projections from the surface, which are sometimes 

 short and inconspicuous, but are often elongated into spines 

 (Plate VIII, fig. 6), presenting a very symmetrical arrangement. 

 These projections are generally formed by the cellulose envelope 

 alone, which possesses an almost horny consistence, so as to retain 

 its form after the discharge of its contents (fig. 436, B, D) ; while, in 

 other instances, they are formed by a notching of the margin of 

 the cell (Plate IX, fig. 1), which may affect only the outer casing, or 

 may extend into the cell-cavity. The outer coat is surrounded 1>\ ,-i 

 very transparent sheet of gelatinous substance, which is sometimes 

 very distinct (as shown in fig. 440 ; Plate IX, fig. 6) ; but in 

 other cases its existence is only indicated by its preventing the con- 

 tact of the cells. Klebs states 1 that in Desmids, as in the other 

 Conjugates, this mucilaginous sheath is composed of two portions a 

 homogeneous substance which is but slightly refringent, and a por- 

 tion which consists of minute rods at right angles to the cell- wall. 

 He regards the sheath as entirely independent of the substance of 

 the cell-wall, and as derived from the protoplasmic contents of the 

 cell by diffusion through the cell- wall. The true cell- wall encloses 

 a parietal utricle, which is not always closely adherent to it ; and 

 this immediately surrounds the endochrome, which occupies nearly 

 the whole interior of the cell, and in certain stages of its growth is 

 found to contain starch granules. The endochrome and starch 

 grains are arranged symmetrically in the two halves of the cell, 

 often in very beautiful patterns, such as bands or stars. 



Many species of desmids have a power of slow movement in the 

 water, the cause of which is not obvious, these organisms being 

 entirely destitute of vibratile cilia. Klebs 2 describes this movement 

 as being of four kinds, viz. : (1) a forward movement on the 

 surface, one end of the cell touching the bottom, wliile the other end 

 is more or less elevated, and oscillates backwards and forwards ; 

 (2) an elevation in a direction vertical to the substratum, the free 

 end making wide circular movements; (3) a circular motion, 

 followed by an alternate sinking of the free end and elevation of 

 the other end ; and (4) an oblique elevation so that both ends 

 touch the bottom, lateral movements in this position, then an ele- 

 vation and circular motion of one end, and a sinking again to an 

 oblique or horizontal position. Klebs regards all these movements 

 as due to an exudation of mucilage, and the first two to the forma- 

 tion during the motions of a filament of mucilage by which the 

 desmid is temporarily attached to the bottom, and which gradually 



1 Untermiclnnnjai fins dem Bot. Inst. Tiibiiiyoi, 1886, p 33H. 

 - li/ologi.schcfi Centralblatt, 1885, p. ;->r>;->. 



