THE EVOLUTION OF SEX IN PLANTS. 305 



is commonly considered as lacking certain energy which is supplied 

 by the protoplasm (kinoplasm) of the male cell. The sperm, on the 

 other hand, while replete with energy, is ill supplied with nutritive 

 protoplasm (trophoplasm) characteristic of the egg. The phenomenon 

 of parthenogenesis and the recent work on artificial parthenogenesis in 

 animal types show that the energy in the egg may be liberated and the 

 wheels of development set in motion by the proper chemical and physical 

 environment without the intervention of the sperm. But generally 

 the sperm is required to start the egg into activity, and complete and 

 normal development in most organisms probably requires fertilization. 



Although most of the stages of sexual evolution are presented in the 

 Volvocaceae, one important stage is there lacking which is especially 

 well illustrated among the brown algae and in the Chaetophoraceae. 

 This is the period of transition from the large free-swimming female 

 gamete to the motionless egg, and covers that evolutionary forward 

 step when the female gamete gives up its motility and the independ- 

 ence of its ancestor the zoospore and becomes the passive receptive egg. 



The brown algae (Phaeophyceae) probably developed as an offshoot 

 from the green algae (Chlorophyceae) at a level considerably above 

 the unicellular forms. The lower members of this group are generally 

 well developed filamentous types or more elaborately organized fronds 

 of considerable thickness. Isogamy is general among the lower forms 

 and heterogamy is characteristic of the highest groups (Fucales and 

 Dictyotales). The sexual conditions among the lower groups are 

 especially well exemplified in the genus Ectocarpus, whose species 

 present a remarkable series of stages illustrating sexual evolution. 

 The gametes are biciliate cells produced in peculiar types of gametangia 

 which are modified branches whose cells become divided into an im- 

 mense number of cubical compartments, each of which develops a 

 single gamete. This type of structure is peculiar to the brown algae 

 and is called the plurilocular sporangium or gametangium. It is one 

 of the most interesting organs in the whole group of the Thallophytes 

 because its structure throws light on the difficult problem of the origin 

 of the archegonium. 



Some species of Ectocarpus have gametes so nearly the same size 

 as to be indistinguishable, but the genus as a whole is remarkable for 

 certain variations in this particular. The plurilocular sporangia are 

 frequently of three distinct sizes, and the zoospores that come from 

 them vary accordingly. It is not always necessary that these zoospores 

 from plurilocular sporangia conjugate in order to grow. There is a 

 great amount of parthenogenetic development depending on environ- 

 mental conditions that are in part understood. Thus cool temperature 

 and bright illumination tend to suppress sexuality in these zoospores, 



vol. lxii. 20. 



