132 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



residue and became deposited as limestone. Limestones believed 

 to have been formed in this manner by Algae occur in extremely 

 early rock formations, and apparently such deposition of calcium 

 carbonate as a result of algal activity has been going on ever since. 

 However, owing to such features as their generally soft bodies. Algae 

 often leave no traces of their original cellular structure, and so as 

 fossils they are difficult to recognize with certainty. In any case 

 there seems no doubt that, well before the end of the Cambrian 

 period, not only an abundance of Blue-green but also large numbers 

 of Green and some Red and probably Brown Algae had evolved. 

 Certainly all of the first three groups were plentiful in the Ordovician. 

 Two examples of fossil Algae of the Cambrian period are shown in 

 Fig. 29, E and F. Although Desmids and even Stoneworts are now 

 known to go back into the Palaeozoic, being present in the Devonian 

 period, the Diatoms appear to be of more recent origin, none being 

 known before the Jurassic period in the middle of the Mesozoic era. 

 It may well be that many groups of Algae have remained evolution- 

 arily static throughout the time since their remote ancestors laid 

 down some of the earliest fossils of which we have knowledge. 



It was indicated above that the Fungi as a group are also very old, 

 and there is no reason to doubt that they have acted as scavengers 

 throughout their long geological past, even as they act today. Indeed 

 it appears that fossils were chiefly formed when deposition of plant 

 material took place under conditions unfavourable to fungal growth, 

 so that the usual destructive activity of Fungi was evaded. Well- 

 preserved fungal mycelia and spores have been found in the tissues 

 of vascular plants as far back as the Devonian, and Fungi apparently 

 occur in sedimentary deposits of much earlier date {see p. 130). 

 Such discoveries have not, however, shed any clear light on the 

 origin of the Fungi, which have long been believed to have evolved 

 from Algae through the loss of chlorophyll. However, some authori- 

 ties now hold that Fungi were derived from a distinct group of 

 primitive organisms, the similarities with Algae being due to parallel 

 evolution, and it may well be that, unlike the various members of 

 most other groups. Fungi had no common starting-point but origi- 

 nated at diiTerent times and in various groups. Fossils of higher 

 Fungi^do not appear with certainty until the Cretaceous, and no 

 indubitable early fossils of Lichens are known. 



The Nematophytales comprise an extinct group of spore-producing 

 Silurian"and Devonian plants of uncertain relationship, in which the 

 plant body was composed of a system of interlacing tubes. It may 



