farr: cytokinesis of pollen-mother-cells 259 



later Strasburger (67a) worked out the details more carefully. 

 There is a ring-like girdle of cell-wall material which grows cen- 

 tripetally across the middle of the cell. As in Spirogyra the ring is 

 margined by a circle of dense protoplasm. In Ulothrix the same 

 author (67a) presents an example of a uninucleate cell which after 

 karyokinesis forms its cross wall entirely centripetally. There is 

 no doubt an excellent field for research on the cell-division of these 

 filamentous green algae. 



In the brown algae several instances have been reported of a 

 division of the protoplast simultaneously across the equator of 

 the cell. This perhaps should not be taken to mean that the cell- 

 division takes place suddenly, but rather that at any one moment 

 the process is at the same stage of development in all parts of the 

 equatorial plane. Strasburger (67^) and more recently Swingle 

 (68) have investigated the process in Sphacelaria. Swingle finds 

 that cell-division is first indicated by the transverse arrangement 

 of the alveoli in the equatorial plane between the daughter nuclei. 

 A number of granules may appear in this region, and presently a 

 fine line is noticed forming a bridge across the entire cell, or some- 

 times only across a part. A cellulose wall then appears. The 

 author is unable to relate this to the spindle fibers which are con- 

 sidered insufiftcient in number, and do not multiply during the 

 processes just described. It is concluded that the process is not 

 dependent upon the activity of the kinoplasm, but, nevertheless, 

 may be under the control of the nuclei. 



Mottier (446) found a similar manner of cell-division in Dictyota. 

 Neither Swingle nor Mottier present in figures any stages in the 

 development of the partition. They each give one figure, and 

 supplement this by description. While the latter is valuable, it 

 cannot replace an accurate reproduction of the evidence at hand. 

 Until some one presents an adequate series of stages of the process 

 we must continue to regard their evidence as not entirely con- 

 vincing. In fact it may even be questioned as to whether we 

 should apply the term cell-plate to these various sorts of forma- 

 tions in the lower plants; it seems desirable rather to restrict 

 the term to that perfectly definite sort of structure that Timber- 

 lake and others have described, and which seems to be so prevalent 

 in the higher plants. 



Farmer and Williams (18a + b) have studied the division of 



