Ponds and ditches 427 



and many others. Many of the arctic types are alpines in lower latitudes, 

 but others (such as Xanthidium groenlandicum) are strictly arctic. Diatoms 

 are also influenced in the same way. Navicula alpina is strictly a subalpine 

 or montane species in England, but occurs in great abundance in the 

 Sphagnum-bogs of the Orkneys and Shetlands almost at sea-level. 



In the tropics most of the boggy areas are at considerable altitudes and 

 they contain many of the ubiquitous types of desmids and diatoms so 

 frequent at lower altitudes in temperate regions. In the lowland swampy 

 places of the tropics desmids are often abundant, but the species are to a 

 great extent very different from those of temperate areas and also much 

 more numerous. Quite a number are more or less ubiquitous throughout 

 tropical areas, but very many are certainly restricted to definite geographical 

 areas. 



In his observations on the alga-flora of Ceylon, Fritsch ('07 A and B) 

 states that the filamentous types of desmids occur mostly in the lowland 

 pools and he is inclined to attribute this fact to the lower percentage of 

 dissolved oxygen. It seems probable, however, that other factors are con- 

 cerned in the production of this filamentous tendency. For instance, in the 

 British Islands the largest number of filamentous desmids occurs in the 

 subalpine lakes of the Welsh and Scottish mountains, and, moreover, under 

 conditions of great aeration. 



C. Algal Associations of Ponds and Ditches. 



Ponds and ditches, being only very small sheets of water, exist under 

 very varied conditions and they differ much among themselves in their alga- 

 flora. All exhibit a decided periodicity and irregular factors have much 

 more effect on the alga-flora than is the case in large sheets of water. 



The Zygnemaceas are represented more especially by species of Spirogyra 

 and Mougeotia. Various species of Zygnema occur in an irregular way, but 

 on the whole in small bodies of water this genus only occurs abundantly in 

 rocky pools or where there is a considerable amount of peat. The various 

 species of Spirogyra differ in their periodicity. Some are purely vernal, 

 whereas others may be both vernal and autumnal. The autumnal phase has 

 been stated to be the result of the germination of a certain percentage of 

 the zygospores formed in the spring (Fritsch & Rich, '07), but in many 

 instances it results from the persistence through the summer of a number of 

 vegetative filaments of short length and few cells. In temperate countries 

 there is no doubt that conjugation takes place mostly in the vernal phase 

 and depends upon a combination of recurring factors. Mougeotia is more 

 particularly a vernal type in ponds and ditches, and reproduction occurs 



