THE GREEN ALG^E 637 



larger eye-spots than the others, and this feature is most 

 strikingly developed in Volvox, where the cells at the posterior 

 end of the colony are practically destitute of eye-spots. In 

 Pleodorina (Kofoid 42, Merton 49) and Volvox again we have 

 the relegation of asexual reproduction to certain larger cells 

 at the back end of the hollow 7 spherical colony, these cells 

 being devoid of cilia in the case of the latter genus. In Pleo- 

 dorina califomica half the cells of the colony are thus destined 

 for reproductive purposes, while in P. illinoisensis and Volvox 

 the number is far smaller. This distinction of somatic and 

 vegetative cells is one of the most important steps in the 

 evolution of the Volvocaceous series, for with it comes the 

 phenomenon of death, a certain portion of each colony ceasing 

 to exist after its life-cycle is accomplished. In Volvox we also 

 have the restriction of the sexual function to certain cells of 

 the colony, a marked step in advance of Pleodorina and Eudorina, 

 in which every cell has the power of forming sexual elements. 

 Hand in hand with the division of labour just noticed goes a 

 gradual evolution of oogamy, commencing with traces of aniso- 

 gamy in Pandorina (Pringsheim 57), becoming much more 

 pronounced in Pleodorina (Merton 49) and Eudorina (Goebel 28), 

 and culminating in the ova and spermatozoids of Volvox. In 

 this latter genus we have therefore arrived at a stage of differ- 

 entiation which is only found among the highest representatives 

 of other lines of algal descent. It was perhaps the conservative 

 tendency shown by the vegetative structure of the Volvocaceous 

 colony that precluded further development along these lines, 

 for it is difficult to conceive how Volvox could be further ela- 

 borated. It is interesting to notice that the Gonium-series is 

 the successful one at the present day as regards abundant 

 occurrence, for both Spondylomorum and Stephauosphcera are 

 very rare forms as compared with the others. 



Returning again to Chlamydomonas, we have now to follow 

 up evolutionary series in which the power of independent 

 movement becomes restricted. This evidently took place along 

 various lines, of which we shall only be able to notice the more 

 important. We first have what we may call the dendroid series, 

 constituted by Oltmann's family, the Chlorodendracese (Olt- 

 manns 53, p. 136). The genus Chlorodendron (fig. 2, c) consists of 

 elongated cells, situated at the ends of hyaline stalks and united 

 to form a branched colony. The cells have the usual basin- 



