34 INTRODUCTION. 



of them ; so that if we examine any species in which this law 

 is known to exist, we shall observe, first, that the filaments 

 differ considerably in diameter in the same specimens ; se- 

 condly, if the species be a branched one, that the largest 

 filaments are near the centre of the specimen ; and, thirdly, 

 that the diameter of all the filaments, whether they be near 

 the centre or circumference, gradually decreases from base 

 to apex ; the observation of these three facts proving the 

 existence of the law of the lateral developement of cells, 

 and also showing it to be in the degree of their age. The 

 proportions of a specimen of a branched Conferva, therefore, 

 are in miniature those of a tree or shrub. This law has no 

 existence in the conjugating Conferva in the genera Bulbo- 

 chate and Desmidium, and in the majority of Oscillatoria. One 

 or two other observations still remain to be made in re- 

 ference to cells, to complete the interesting subject of the 

 developement of the Conferva. Prior to the discovery of the 

 multiplication of cells by division, the opinion was generally 

 entertained that each cell was at its commencement very 

 small, and that it increased little by little, until it attained 

 its perfect developement ; and this is the case with the cells 

 of parenchymatous tissue ; but in the examples where the 

 multiplication of cells takes place by division of those cells 

 which have already become great, nature takes a means for 

 the fulfilment of its end, altogether different, but not less 

 effectual. 



It has already been mentioned that the different series of 

 cells of which the Conferva are composed, are all enveloped 

 in a continuous membrane, which serves to bind them to- 

 gether; there would appear, however, according to the re- 

 searches of Agardh and Hugo Mohl, to be another and direct 

 bond of union between the cells, not merely of the Algae, 

 but of all other plants, this consisting in an adhesive sub- 

 stance which Mohl has denominated " intercellular," by which 

 they are firmly united to each other. The same substance is 

 found coating the surfaces of the pollen granules, binding 

 them into masses. 



Having thus traced the developement of the Conferva from 



