XANTHOPHYCEAE 119 



(macrocyst) or into several multinucleate spores (sporocysts), or 

 else the contents migrate to the rhizoids and there form several 

 cysts (rhizocysts) which, when conditions are again favourable, 

 either germinate directly to a new plant or else give rise to zoo- 

 spores. In B. granulatum it is estimated that about 40,000 iso- 

 gametes are formed in each vesicle, but as the plant is monoecious 

 many fuse either in pairs or threes, rarely fours, before they are 

 liberated. Those that do not fuse develop parthenogenetically, 

 although the stage at which meiosis occurs is not yet known. The 

 life cycle can be tentatively represented as in fig. 83 D. 



*BACILLARIOPHYCEAE (DIATOMACEAE) 



Figs 84, 85 



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 

 mucilaginous 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 

 (hypotheca), each half being composed of a valve together with a 

 connecting band, the latter forming the overlapping portion. The 

 Diatomaceae are divided into two groups, the Pennatae and Centri- 

 cae, the former having intercalary bands as well as the connecting 

 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 

 microscopes. The Pennatae have the striae arranged in series with 

 either a plain area in between (pseiidoraphe) or else a slit that varies 

 in form and structure (raphe). In the Centricae these structures are 

 absent and the striae are arranged radially. The raphe is connected 

 with movement, as only those forms possessing one have the power 



