202 



KNOWLEDGE. 



June, 1913. 



branches from the stock, which have undergone 

 comparatively little modification, and which 

 terminate in living species. The real direct line is 

 from P, the primitive type, to G, while the so-called 

 evolutionary series is represented by the forms 

 A to G. The conception must usually be under- 

 stood in this sense, and the Volvox line con- 

 sidered below may be cited as a concrete example. 



Collective evidence has shown that the main lines 

 of progressive evolution in the Algae have probably 

 diverged from a group represented to-day by the 

 Flagellata, a complex of minute organisms lying on 

 the border line between the animal and vegetable 

 kingdoms. These Flagellates, as their name indicates, 

 are ciliated free-swimming " creatures," often sapro- 

 phytic in their mode of life, of which some members 

 are colourless, some possess chlorophyll alone as a 

 pigment, while others are coloured yellow, brown or 

 red by substances closely allied to or often, perhaps, 

 identical with those present in the brown and red 

 sea-weeds. There is no need to consider these 

 organisms further here. 



Those forms, which are unicellular and motile 

 throughout the greater part of their life-history, seem 

 to illustrate the types of Algae which are nearest to 

 the primitive forms and to their hypothetical pro- 

 genitors, the Flagellata. Considering for the present 

 the green Algae or Chlorophyceae alone, such forms 

 are found in the order Chlamydomonadaceae (of 

 which the genus Spluwrella is a well-known type), 

 and various authors have taken the genus Chlamydo- 

 monas as a primitive type, and have outlined systems 

 of classification indicating the probable lines of 

 evolution from this. 



Chlamydoinonas, or the common pool-alga Sphae- 

 relhz, which is very similar, is unicellular, with a 



yrtt-yt 



single nucleus and chloroplast, and possesses two 

 fine protoplasmic cilia attached to a small beak-like 

 projection at the anterior end of the cell. It is 

 motile by means of these cilia throughout the 

 greater part of its life-history. Reproduction takes 

 place by simple fission of the cell contents, or by 

 conjugation of gametes. Under unfavourable con- 

 ditions, the cells may go into a non-motile resting 

 condition. 



Starting from such Chlamydoinonas forms, three 

 main series may be recognised among the lowlier 

 Algae leading to the establishment of three distinct 



A 



»*> 



3 



Figure 207. Some forms of Volvox Line. Diagrammatic, 

 a. Chlamydoinonas. b. Goniutn. 



c. Pandorina. 



d. Pleodorina. 



£>--:_<:' •---«L 



Figure 208. Diagram illustrating the evolution of 

 forms. 



types of thallus : one to the formation of colonial 

 bodies, one to the filamentous and plate-like 

 forms, and the third to the so-called siphoneous or 

 coenocytic type of plant body. The organisation of 

 these various types of plant body will be more fully 

 described when we consider the several lines in 

 detail. 



The Volvocineae comprises a series of genera and 

 species which have long been recognised as closely 

 related, and they seem to illustrate well the stages 

 which have probably led to the development of a 

 free-swimming colony. In Gonium there is an 

 association of four or sixteen cells, each like 

 Chlamydomonas provided with two cilia, the whole 

 colony being in the form of a flat square (see 

 Figure 207, b). There is a non-motile resting period 

 when asexual reproduction takes place — simply by 

 the division of each cell to form a new colony. 

 Pandorina is slightly more complex ; there are six- 

 teen cells arranged in the form of a closely packed 

 sphere. The cells are all alike, each with its two 

 cilia, and any cell of the plant can divide 

 to produce a new colony or coenobium as it is called. 

 Pleodorina has the cells at one end wholly vegetative 

 and smaller than the rest — the first real indication of 

 the structural unity of state — and the sterile cells 

 represent an incipient vegetative body or " soma " 

 as opposed to the portion concerned in reproduction 

 alone (see Figure 207, d). Pandorina is quite 

 common in freshwater pools and ditches in this 

 country, but Pleodorina does not seem to occur. 

 The culminating type of the line is Volvox, well 

 known to all microscopists. The colonies here 

 consist of several thousand cells arranged in the 

 form of a hollow sphere, and there is a definite 

 sterile pole. There is a fairly high type of 

 sexual reproduction besides the asexual method by 

 division of the parthenogonidia. The unit}' of the 

 whole colony is still more pronounced than in the 



