i8 



FIRST GROUP. — THALLOPHYTES. 



of the Diatomaceae. Besides the ordinary rotation of protoplasm in their interior, 

 Diatoms exhibit a creeping movement, by means of which they glide over fixed 

 bodies or push small granules that are near them along their surface ; this occurs 

 only in a line drawn along the length of their wall, in which Schultze supposes slits 

 or holes, through which the protoplasm protrudes; this has never been directly 

 observed, but it is perhaps the cause of the gliding movement, which is referred by 

 others to osmosis '. 



I'lG. 9, 



(front view 

 endochrome 



if<haerofhora slinwint; the surface of a 

 e (side view), and ri^lit and left the girdle 

 the right and left margins of the valves; the 

 shaded ; magn. 900 times. After Pfitzer. 



Fig. 10. Go))tphoiieina constyictuju Ehrbg. ; s view oi 

 valves (nucleus visible) ; g, view of the girdle corresponding 

 to the right, s„ to the left margin of the valves ; q transverse 

 section through the middle of the cell, showing with unusual 

 distinctness the composition of the silicified cell-wall of two 

 halves, one overlapping the other (jir the larger, si the smaller 

 valve) ; k the nucleus ; / dense protoplasm ; gf g„ the two 

 girdle-surfaces. After Pfitzer. 



The cells of the Diatomaceae live in fresh, brackish and salt water, either single or 

 united in rows ; some are borne either singly or a number together on gelatinous stalks 

 (Fig. 10), or they are imbedded in a gelatinous mass, which in some species takes the 

 form of regularly branching threads, as in Schhonema. They multiply by bipartition. 

 When cell-division begins the two valves move apart from one another, and the 

 contents divide into two daughter- cells, each of which forms a new valve on the plane 

 of division ; the inilexed margin (the girdle) of the new valve fits into the girdle of the 

 old valve of the mother-cell ; the old valve extends beyond the edge of the new valve, 

 as a lid over the edge of a box. The two new valves of the two daughter-cells lie at 

 first in contact with one another. Since, according to Pfitzer, the valves, which though 

 highly silicified contain at the same time some organic matter, do not increase in size 

 after formation, it is plain that the new ones must be smaller and smaller from 

 generation to generation ; when their size has reached a certain minimum, large cells, 

 the auxospores, are again suddenly produced ; the valves of the small cells separate, 

 and the contents issue forth and increase in size by growth, or by conjugation and 

 growth combined ; the auxospores thus formed provide themselves with new valves. 

 Since the large auxospores are somewhat differently shaped from their smaller mother- 

 cells and primary mother-cells, cells differently shaped and with unlike lobes must 

 necessarily result at first from their division, as happens also with the Desmids. The 

 two valves moreover are always of unequal age. 



As regards the mode of formation of the auxospores five types may be distinguished, 

 in accordance with the views of Pfitzer and Schmitz. 



The simplest type is seen in a simple rejuvenescence of single cells. An individual 



See Mereschowsky in Bot. Ztg. 1880, p. 520. 



