Profosiphonaceae 223 



they are of course mostly located in the more peripheral branches. In some 

 of the Siphonales the nuclei are not infrequently larger than the chloroplasts. 



Multiplication occurs in some forms by larger or smaller proliferous shoots 

 which become detached, or by mere fragmentation of the thallus. Asexual 

 reproduction takes place by non-motile gonidia (aplanospores), and by 

 zoogonidia which arise in considerable numbers in zoogonidangia. In 

 Vaucheria the zoogonidium is a large compound structure and is solitary, 

 escaping from a terminal zoogonidangium. Gamogenesis occurs in some 

 forms by the fusion of isogametes or anisogametes. In the family Vaucheriacea? 

 there are highly differentiated sexual organs. 



It is not unlikely that the Siphonales have been evolved from the Proto- 

 coccales by a further extension of the ccenocytic forms of the Chlorococcinese. 

 The genus Protosiplton is an example of an Alga which, after all, is not very 

 far removed from Phyllobium among the Chlorochytriese, and it may possibly 

 be a relic of those early forms in which originated the tendency which 

 finally led to the Siphonales. It is exceedingly probable that Halicystis is 

 the type from which Valonia has originated. 



The view put forward by Oltmanns that the Siphonales are probably 

 derived from the Siphonocladialcs by the suppression of the septa, although 

 deserving of consideration, suggests a line of evolution which appears very 

 improbable. 



There are a number of undoubted fossil siphonaceous Alga?, the calcified 

 thallus of various forms having lent itself to preservation. 



Family Protosiphonacese. 



The principal genus of the family is Protosiphon Klcbs (1896), which 

 appears to be the simplest known form of the Siphonales. The crenocytes, 

 which, like those of Botrydium, live on damp earth, are bladder-like or 

 subtubular (averaging about 100 /j, in diameter) with a narrow, elongated, 

 rhizoidal extension passing down into the soil. There is a single reticulate 

 parietal chloroplast containing several pyrenoids. Starch is the principal 

 food-reserve, and numerous nuclei occur in the lining layer of cytoplasm. 

 The plants multiply by the budding off of new individuals from the green 

 part of the older parent-coenocytes, or by the division of the young coenocytes 

 into 4, 8, or 16 daughter-ccenocytes. Under certain conditions the proto- 

 plasmic contents become divided into a number of rounded ' cysts ' (probably 

 aplanospores) which turn red and rest for a time. They may germinate 

 direct into new plants or they may form biciliated gametes. The ordinary 

 vegetative coenocytes may also become gametangia and produce ovoid 

 isogametes like those formed from the aplanospores. The gametes fuse in 

 pairs, producing a star-shaped zygote. After a period of rest the zygote 

 grows direct into a new vegetative individual. Gametes which do not 



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