362 BOTANY OF THE LIVING PLANT 



to the motile condition. In the simpler forms the individual consists 

 of a single ciliated cell : in the more complex forms many such cells 

 are aggregated together. The fact that the Volvocales are sometimes 

 claimed by zoologists shows how closely the two Kingdoms are related 

 by these primitive creatures. A simple example of them is seen in 

 Sphaerella (Haemato coccus) pluvialis, a unicellular organism frequent 

 in water-butts and puddles (Fig. 267). Its motile stage is an oval 

 cell (A) with a protoplast containing a large chromatophore, and 

 several pyrenoids. It is surrounded by a mucilaginous cell-wall, 

 through which two cilia project. It readily becomes encysted forming 

 " Red-snow " in high latitudes, which is its " Palmella " stage. 

 From this, after division (B), the contents escape as motile zoospores, 

 which may grow and again divide. Gametes are formed by division 

 of a mother-cell into 8, 16, or 32, which escape as motile isogametes 

 all alike (D), and conjugate in pairs (E) to form encysted zygotes 



(F,G). 



Chlamydomonas has a similar life-history. In some species sexual 

 reproduction takes place by the fusion of isogametes whereas in others 

 the gametes are of different size (heterogametes). In the latter a 

 fertilisation of the larger female gamete (egg) by the smaller male 

 gamete (spermatozoid) occurs. The zygote undergoes a reduction 

 division on germination. 



The sex-difference thus indicated in Chlamydomonas is much more 

 marked in Volvox globator. This organism appears in ponds or pools 

 of fresh water, in the form of hollow free-swimming spherical colonies 

 (Fig. 268, A). Each is composed of a film of cells embedded in 

 mucilaginous cell-walls, through which pairs of cilia protrude on the 

 outer surface. The whole colony shows a slow rolling movement 

 due to their activity. It propagates vegetatively by subdivision 

 of certain larger cells, each forming a daughter-colony within the 

 parent, which are set free by its disorganisation. A greater interest 

 attaches to the sexual propagation. Large non-motile female cells 

 ( e gg s ) project into the cavity (D) and are there fertilised by minute 

 motile spermatazoids produced by subdivision of antheridial cells (B). 

 A thick-walled resting zygote is the result, which may germinate to 

 form a new colony. 



This series of members of the Volvocales illustrates steps in the 

 origin of sex. In Sphaerella the gametes are all alike. In Chlamy- 

 domonas they are alike in some species, but unequal in others in point 

 of size. In Volvox that inequality is more marked, and the behaviour 

 of the large inactive egg is contrasted with that of the minute and 



