186 Chlorophycece 



Plants of this family sometimes occur in prodigious quantity 

 in stagnant water, giving it a pale-green colour and a somewhat 

 unpleasant odour. They are occasionally the cause of foulness of 

 drinking-water, imparting to it a distinct oily taste 1 . 



Excluding the Polyblepharideee, which exhibit a mixture of Flagellate and 

 Volvocine characters, the British Volvocacese are divided into the three fol- 

 lowing sub-families 2 : 



Sub-family I. Chlamydomonadece. Unicellular ; globose or ovoidal, 

 with a distinct but thin cell-wall. Cilia (or flagella) two, rarely four. 

 With one chloroplast of very variable form, usually including a single 

 pyrenoid. 



Sub-family II. Phacotece. Unicellular, with the cells as in the 

 Chlamydomonadese, but with a thick solid cell-wall which separates 

 into two halves on the escape of the daughter-cells. 



Sub-family III. Volvocece. Motile coenobia of Chlamydomonadiue 

 cells, usually embedded in a common mucilaginous investment ; more 

 rarely united by protoplasmic processes. All the cells are capable of 

 reproducing the plant or there is a differentiation into vegetative and 

 reproductive cells. Vegetative multiplication by division of some or 

 all of the cells to form daughter comobia. Isogamous or heterogamous 

 sexual reproduction. 



Sub-family I. CHLAMYDOMONADESE. 



This division of the Volvocacese includes a number of unicellular 

 Algae which are spherical, ovoid, subcylindrical, or rarely fusiform 

 in shape, and are provided with a thin cell-wall and two, or more 

 rarely four, cilia. The chloroplast is of variable form, but is 

 typically cup-shaped and occupies the posterior region of the cell, 

 more or less surrounding a central mass of protoplasm which 

 contains the nucleus. There is usually one pyrenoid, and a lateral 

 pigment spot is also generally present in the anterior region of 

 the cell. 



Reproduction takes place by the division of the contents of a 

 cell which has come to rest into 2, 4, or 8 daughter-cells, each 

 of which soon acquires the characters of the mother-cell. The 

 successive division-planes are at right angles to each other and 

 the daughter-cells are ciliated motile individuals. Sometimes the 

 vegetative cells assume a palmelloid condition, in which they 



1 Whipple in Trans. Amer. Micr. Soc. xxi, 1900. 



: Tliis classification is after Dill, 'Die Gattung Chlamydomonas und ihre 

 iitichsten Verwanclten.' Jahrbiicher fiir wissenschaftliche Botanik. Berlin, 1895, 

 Bel xxviii. 



