VIl] SCARCITY OF FOSSIL ALGAE. 141 



activity of a zone of dividing or meristematic elements. The 

 occurrence of distinct concentric rings in this secondary tissue 

 clearly points to some periodicity of growth which is expressed 

 by the alternation of narrow and broader cells. In the Ant- 

 arctic genus Lessonia, the stem reaches a girth equal to that of 

 a man's thigh, and in structure it agrees closely with the smaller 

 stem of Laminaria. In these large algal stems, the cells are 

 not lignified as in woody plants, and in longitudinal section 

 they have for the most part the form of somewhat elongated 

 parenchyma, differing widely in appearance from the tracheids 

 or vessels of woody plants. At the periphery of the Laminaria 

 stem, represented in fig. 29, there occur numerous and com- 

 paratively large mucilage ducts. 



In certain algae of different families the thallus is encrusted 

 with carbonate of lime, and is thus rendered much more 

 resistant. The Diatoms, on the other hand, possess still more 

 durable siliceous tests which are particularly well adapted to 

 resist the solvent action of water and other agents of destruction. 

 It is these calcareous and siliceous forms which supply the 

 greater part of the trustworthy data furnished by fossil algae. 



It remains to consider some of the causes to which we may 

 attribute the scarcity of fossil algae, and the possible sources 

 of error which beset any attempt to describe or assign names 

 to impressions and casts simulating algal forms. 



In the first place, the delicate nature of algal cells is a 

 serious obstacle to fossilisation. Even in plants in which the 

 woody stems have been preserved by a siliceous or calcareous 

 solution, we frequently find the more delicate cells repre- 

 sented by a mass of crystalline matter without any trace of the 

 cell-walls being preserved. In such plants as algae, where 

 the cell-walls are not lignified, but consist of cellulose or some 

 special form of cellulose, which readily breaks down into a 

 mucilaginous product, the tissues have but a small chance of 

 withstanding the wear and tear of fossilisation. 



The danger of relying on external form as a means of 

 recognition is especially patent in the case of those numerous 

 markings or impressions frequently met with on rocks, and 

 which resemble in outline the thallus of recent algae. Among 



