222 Siphonales 



formed zoogonidia if exposed to light, but gametes if kept in darkness. If, however, the 

 cosnocytes had a strong disposition to form zoogonidia, then zoogonidia were formed in 

 both light and darkness, and similarly ccenobia with a strong disposition to gamete- 

 formation formed gametes whatever the nature of the illumination. The contest between 

 the propensities toward zoogonidium-production and gamete-production appears to be 

 perpetual, and the balance between the operative stimuli must be very delicate. 



Quite recently another species, H. africanum, has been described by Yamanouchi ('13). 

 This had developed in a culture of some soil obtained from near Cape Town, S. Africa. 

 In this species the ccenocytcs are for the most part ellipsoidal and about 60 of them form 

 a very irregular net. They have a remarkable turgidity, and the connection between them 

 in the larger plants is by such a small portion of the surface that the co3nobium cannot 

 be moved without breaking it. In the final stages of development the cccnocytes become 

 solitary, almost globular, and attain a diameter of 1'5 centimetres. In this species there 

 are numerous definite chloroplasts, in which there are pyrenoids and reserve starch -grains, 

 but the latter are not formed by the direct fragmentation of the pyrenoids as described by 

 Timberlake in H. reticulatum. 



Order 2, SIPHONALES. 



The Siphonales include a number of Green Algse of very diverse form, 

 which all agree in the fact that the entire plant consists of a single ccenocyte, 

 even though it may have a complex structure and attain a length of 40 cms. 

 True septa only occur in connection with the formation of reproductive 

 organs, although secondary septa of a peculiar kind may arise in the life of 

 some of the forms. Except for the genera Protosiphon, Phytophysa, Phyllo- 

 siphon, Dichotomosiphon, and about half the species of Vaucheria, all the 

 forms are marine, and to a great extent inhabitants of warm seas. 



The simplest type is Protosiphon, in which the primitive rounded form of 

 the cell is largely retained. In the Vaucheriacese and Phyllosiphonacese the 

 thallus is filamentous and somewhat branched ; in Bryopsis and Caulerpa the 

 branching is much more extensive and exhibits a wonderful symmetry; in 

 Halimeda, Penicillus, and others the thallus is a dense aggregation of 

 Branches bound together by a deposit of calcium carbonate ; in Codium the 

 branches are elongate hypha-like filaments which are densely interlaced to 

 form a branched thallus of some solidity. It is this extraordinary complica- 

 tion of a single coenocyte which is the distinguishing feature of most of the 

 Siphonales ; and of the known forms, those embraced in the genus Caulerpa 

 may be regarded as amongst the most remarkable of all Green Algae. The 

 plants are in nearly all cases attached by strongly developed hold-fasts (haptera). 



The protoplasm forms a parietal layer within the wall of the co3nocyte 

 and contains numerous nuclei. Thus, the whole thallus consists of many 

 protoplasts which are not individually separated by cell- walls but are all 

 enclosed within a common wall. Numerous disc-shaped or lens-shaped chloro- 

 plasts occur in the lining layer of protoplasm, although in the complex forms 



