Ponds and ditches 429 



Cladophoracese fur life in tropical waters, which owing to their higher 

 temperatures contain less dissolved oxygen and carbon-dioxide. It is also 

 possible that the peculiar resting- spores so characteristic of this genus enables 

 it to live in situations liable to rapid desiccation. The Cladophoracete of 

 ponds and ditches are generally infested with epiphytes. In temperate 

 climates these are largely diatoms of the genera Cocconeis (consult fig. 84), 

 Epithemia, Rlioicosphenia and Achnanthes. In the tropics the filaments of 

 Pithophora often carry epiphytes, the most important being a number of the 

 smaller species of (Edoyonium (generally with depressed-ellipsoid basal cells) 

 and several species of Endoderma (consult fig. 194 D and E). 



Many species of CEdogonium occur in ponds and ditches, and in the 

 larger ponds a few species of Bulbochxte may be found. They are either 

 spring or summer types, depending upon the species, although not a few are 

 found far into the autumn. Sexual reproduction depends very largely upon 

 meteorological conditions and in most cases does not occur every year, the 

 plants being reproduced asexually until the exact conditions for sexual 

 reproduction supervene. The sexual organs may be produced any time 

 between April and September. The filaments are sometimes covered with 

 species of the epiphytic genus Charadum, and Aphanoch&te repens is often 

 a common epiphyte. 



Of the Ulvacese only Enteromorpha intestinalis is at all frequent, although 

 at least two species of Monostroma occur in small ponds. 



Of the Protococcales the Volvocacese deserve the first mention. The genera 

 Garteria, Chlamydomonas, Pandorina, Gonium, Eudorina and Volvox all occur 

 abundantly ; Eudorina is not uncommon, but Pleodorina is decidedly rare. 

 The various species of these genera are to a great extent erratic in their 

 occurrence, but on the whole they are cold-water types. Species of Chlamydo- 

 monas are the most numerous and some of them may be found at any time 

 between September and May in the active motile condition. The Auto- 

 sporaceae are well represented by Scenedesmus, Ankistrodesmus, Selenastrum, 

 Ccelastrum and other genera, and several species of Pediastrum may be 

 abundant. The great majority of the Tetrasporine and Chlorococcine Proto- 

 coccales are late spring and summer forms, often occurring far into the 

 autumn 1 . Genera of the Micractiniea3, which were at one time thought to 

 be largely plankton organisms, often occur in quantity in small ponds. 



In the HeterokontaB Tribonema bombycinum (with its forma minor) is 

 often abundant and not infrequently mixed with Microspora floccosa and 



1 Delf ('15) writing on the 'Algal Vegetation of Ponds on Hampstead Heath,' London, states 

 that the majority of the Protococcales are early spring forms attaining their maximum develop- 

 ment in the month of March, but most of the genera mentioned by this author belong to the 

 Volvocine series, a group in which the majority of the species have a preference for comparatively 

 cold water, although they are often very erratic in their appearance. 



