24 PLANT LIFE. 



undergoing. From geological evidence it would appear 

 that these very large cells constituting individuals were 

 much more general in long bygone times than at the present 

 day ; in fact during the early geological periods, so far as 

 algae are concerned, it appears that the morphological differ- 

 ence between very minute and very large seaweeds consisted 

 in the relative size and differentiation of a single cell rather 

 than in the aggregation of numerous individually minute 

 cells as is usual at the present day ; although we have yet a 

 small remnant of this antiquated group of large unicellular 

 forms. Undoubtedly many fossils or impressions in the 

 rocks that were at one time considered as the remains of 

 gigantic unicellular algae have been proved to be due to 

 other causes ; yet there appears to be distinct evidence of 

 the existence of such in early geological times. 



Contemporaneous with the development of the configur- 

 ation of the cell due to local growth of the cell-wall, the 

 latter usually also increases in thickness, and in some 

 cases to such an extent as to almost completely fill up the 

 cavity of the cell. Increase in thickness takes place by 

 additions to the inner surface of the cell-wall, and again 

 this thickening matter is very rarely uniformly deposited, 

 certain portions of the original thin wall being left unthick- 

 ened. As the local deposition of secondary thickening 

 matter is constant within narrow limits in corresponding 

 groups of cells in all the individuals of a species, or even 

 in large groups of plants, it follows that the configuration 

 produced by the secondary thickening of the cell-wall is 

 a factor of some importance in indicating affinity, and is 

 especially useful in the case of fossil plants, where the 

 identification of the fragmentary remains depends to a 

 very great extent on their microscopic characters. Amongst 



