24 INTRODUCTION. 



and, as it appears he was not aware that it had previously 

 been noticed by continental botanists, another proof is thus 

 afforded that it has attracted the attention of independent 

 observers. 



The fact of the transverse bisection of the Algse is now so 

 well established that this character is employed by J. G. 

 Agardh, in his generic definitions*. 



In a preceding part of this introduction, I have endea- 

 voured to show that in the Desmidiese the process is the same 

 as in all the simple Algae. 



If some should think that I have dwelt too long upon this 

 topic, I trust I shall stand excused when it is considered that 

 the celebrated Ehrenberg places much reliance upon this 

 character, and that great weight is justly allowed to the 

 authority of one who has devoted so much time and skill to 

 the examination of the Desmidieae ; moreover, that his argu- 

 ments are familiar to the British naturalist, and his opinions 

 extensively embraced, whilst the opinions of those who differ 

 from him on this subject are comparatively unknown. 



I am not in a position either to deny or to affirm with con- 

 fidence the presence of openings in the extremities of the 

 Closteria, for in objects so minute it is very difficult, fre- 

 quently perhaps impossible, to distinguish between a depres- 

 sion and an openingf. It appears indeed to me that in 

 Closterium there is a slight notch, or more usually the rudi- 

 ment of one, at the apex of the segments, a mere indication 

 in short of what is fully developed in Tetmemorus and Euas- 

 trum ; for in C Diance, and some other species, there is an 

 evident though minute notch, and in Penium not even a trace 

 of it can be detected. In no instance can any portion of 

 the contents of the cell be forced out from the extremities. 



* " Tiresias, fronde tota homogenea et articulis omnibus continua subdi- 

 visione iterum itenimque divisis, atque coniocystis inclusis insigne." — Ag. 

 Icon. Alg. Ined. 



t " No one accustomed to the use of the microscope can be otherwise than 

 aware how much very minute objects seen under a high power are apt to as- 

 sume a character in accordance with preconceived notions." — J. T. Smith, in 

 Annals of Natural History, vol. xix. p. 2. 



