448 PRACTICAL BOTANY 



It has recently been shown that the cell-wall of most of the 

 Desmids is composed of two parts which fit together like a pill- 

 box and its lid : it is even stated that in Closterium it consists of 

 four parts, there being two external shells, and two girdles which 

 fit within them, about the equatorial position : this is the ex- 

 planation of the transverse striae above noted, but it requires high 

 powers to recognize these striae with certainty as the margins of 

 the shells and girdles above alluded to. In most of the Desmideaa 

 there is a gelatinous sheath, of greater or less extent, covering the 

 cell-wall externally, similar to that in Spirogyra. 



Other Desmids may also be compared, e.g. Micrasterias, which 

 is to be found in peaty pools : it is of flattened form with a deep 

 median constriction, and stellate outline : the pyrenoids are very 

 well seen in this species. For description of other forms of this 

 family, many of which are of great beauty, reference must be 

 made to books dealing specially with the subject : it should be 

 noted, however, in examining them that in the larger majority of 

 them the cell is divided into two symmetrical halves, and that 

 frequently there is a sharply marked equatorial constriction, as 

 in Micrasterias. 



Conjugation takes place in the Desmids, and it may 

 frequently be observed in Closterium, the main points 

 being as follows: two cells approach one another, 

 usually placing themselves with their longer axes paral- 

 lel, but sometimes at right angles. Processes are then 

 formed from the two cells, as in Spirogyra, which meet, 

 and fusion takes place at the point of contact. Mean- 

 while the protoplasm passes from the ends of the two 

 cells towards their equators, coalesces, and the zygote 

 is formed in central position : it is an oblong cell, which 

 finally becomes rounded, and covered by a thick 

 cell-wall, while the tests of the original cells fall 

 away. 



