INTRODUCTION TO CRYPTOQAMIC BOTANY. 167 



Conferva} are exposed to drought, they sometimes throw down 

 roots from their joints in search of moisture, a circumstance 

 which must be taken into account in the estimation of species. 

 In Lyngbya muralis, the threads often anastomose, producing 

 a very curious and puzzhng appearance. In the Conjugates 

 the threads and their articulations are equally variable, but 

 then there are differences in the disposition of the endochrome, 

 and the formation of the spores, which afford good characters, 

 provided every variation of position and form be not consi- 

 dered specific. In Anabaina and allied genera, the number 

 and disposition of the fertile cells will not afford safe characters, 

 nor will mere microscopic measurement, which is often decep- 

 tive, and should be always taken with considerable latitude 

 amongst Oscillatoria}. The zoospores even of the articulated 

 Algte are not absolutely constant. Monstrous forms occur 

 in the small zoospores of ClaJophora and the large ones of 

 (Edogonium.^ Characters like those in Hassal's Fresh water 

 Algee, dependent simply on comparative size, are altogether 

 inadmissible. 



143. Where plants, as many Vaucherice, grow under very 

 different circumstances, being sometimes quite dry, sometimes 

 covered with water, and that of various depth, the diiference 

 of length and thickness and of the masses formed by the 

 threads will be very great. I have figured in the supple- 

 ment to Eng. Bot. tab. 2841, the aquatic and terrestrial 

 form of V. ccespitosa, Ag., which at first sight seem alto- 

 gether different. Amongst the calcareous Algae, considerable 

 variety occurs in the shape of the frond in widely distri- 

 buted species. Halimeda opuntla, for instance, varies greatly 

 in form and size, and this is probably the case with many others. 

 In Caulerpa it is quite certain that species have been need- 

 lessly multiplied. Decaisne has pointed out one or two cases 

 in his Plants of Arabia. In C. peltata, Lamx., the same plant 

 exhibits peltate branches, and the cylindrical adpressed 

 branches of C. Chemnitzia, Lamx., while in C Freycinetii, 

 Ag., branches strongly toothed and even apiculate {C. serru- 



* Cohn, 1. c. tab. 20 Many other in stances might be quoted. 



