Palmellacede 1 s:> 



with no precise limitations, but in the Selenastreae, Crucigeniere and Coe- 

 lastreae there are definite non- motile ccenobia. In the lower families the 

 cells are of the Volvocine type, but in the higher families they are of various 

 shapes, frequently greatly elongated, and sometimes furnished with spines or 

 bristles. Each cell contains one or more chloroplasts. In the lower types 

 (Palmellaceap, etc.) the single chloroplast is usually of the Chlamydomonadine 

 character, but in the higher types it is of various shapes, and often so massive 

 as to render its exact form very obscure. The pigmented part of the proto- 

 plast is in some forms very indefinite. Both axile and parietal conditions 

 are met with, but when a number of chloroplasts are present in a cell they 

 are usually parietal. A stigma (or pigment-spot) is met with only in the lower 

 forms, where it may occur both in the vegetative and reproductive phases. 



Reproduction may occur by vegetative division of the mother-cell or by 

 the formation of biciliated zoogonidia. In the Autosporacese zoogonidia are 

 entirely absent, although it is possible that the autospores are really zoogo- 

 nidia which have lost the power of movement. In some genera reproduction 

 also occurs by isogamous planogametes. 



In the Chsetopeltidacese the cells are furnished with bristles of various 

 kinds, sheathed or unsheathed at the base. 



The Tetrasporinese includes a number of important families of the Proto- 

 coccales, one of which (the Palmellaceae) is almost certainly on the direct line 

 of descent of the higher Green Algae. 



Family Palmellacese. 



This family is of considerable interest since it illustrates the first step 

 in the Tetrasporine direction ; that is, the formation of a colony which is 

 non-motile in the adult state and which consists of the products of successive 

 vegetative divisions held somewhat loosely in a mass of jelly. The colonies 

 thus formed are of an irregular character, although the particular form 

 assumed may be characteristic of certain species (Tetraspora) or even genera 

 (Palmodictyon, Palmophyllum, etc.). The normal vegetative condition in the 

 Palmellacese is thus quite distinctive, the cells as a rule being irregularly 

 scattered through the mucilaginous mass, more especially near its periphery. 

 In Tetraspora and Palmophyllum the grouping is somewhat more definite, 

 in the former genus the products of vegetative division remaining more or 

 less in aggregates of four, and in the latter the cells assuming a seriate 

 disposition within a tough leaf-like expansion. The colonies are either 

 microscopic (Pcdmodactylon, Spharocystis, Apiocystis) or macroscopic (Te- 

 traspora, Collinsiella), and may attain a length of several centimetres. 



The cells are essentially Chlamydom6nadine in their cytological details, 

 exhibiting in most cases a single parietal chloroplast, often of a cup-shaped 

 character, and furnished with one pyrenoid. In Asterococcus the chloroplast 



