CHRYSOPHYCOPHYTA 269 



number takes place when the zygote germinates, thus indicating 

 that the adult plant is haploid. This character is quite anomalous 

 when contrasted with the known diploid status of the members of 

 the Siphonales, but is not anomalous in respect of the Xantho- 

 phyceae. 



REFERENCES 



Phyllosiphon 



Mangenotj G. (1948). Compt. Rend. Acad. Sci. (Paris), 226 (10), 1103. 



Taxonomy 

 Pascher, A. (1937-9). Heterokonten. In L. Rabenhorst, Kryptogamen- 

 Flora von Deutschland, Oesterreich und der Schweiz. Leipzig. 



Bacillariophyceae (Diatoms) 



Figs. 156-7 



These unicellular algae are abundant as isolated or colonial forms 

 in marine or fresh-water plankton and also as epiphytes on other 

 algae and plants. They form a large proportion of the bottom flora 

 of lakes and ponds and occur widely on salt marshes, although 

 certain diatoms are said to be very sensitive to the degree of salinity 

 in the medium. In the colonial forms the cells are attached to each 

 other by mucilage or else they are enclosed in a common mucila- 

 ginous envelope. The plants have characteristic silicified cell walls 

 which are built up on a pectin foundation and are highly sculptured. 

 Each shell (frustule) is composed of two halves varying much in 

 shape, the older (epitheca) fitting closely over the younger Qiypo- 

 theca), each half being composed of a valve together with a con- 

 necting band, the latter forming the overlapping portion. The 

 Diatomaceae are divided into tv/o groups, the Pennatae and Cen- 

 tricae, the former having intercalary bands as well as the connect- 

 ing bands. A simple way of distinguishing between these two groups 

 is that the Pennatae have the shape of date boxes and the Centricae 

 that of pill boxes. The marks or striae on the frustules are composed 

 of rows of dots which represent small cavities, and these are so fine 

 that they are employed in testing the resolving power of micro- 

 scopes. The Pennatae have the striae arranged in series with either 



