58 ANCIENT PLANTS 



There are, of course, many minor varieties of cells, 

 but these illustrate all the main types. 



Among the early fossils, however, one type of wood 

 cell and one type of bast cell, so far as we know, are not 

 present. These cells are the true vessels of the wood 

 of flowering plants, and the long bast cells with their 

 companion proteid cells. The figure of a metaxylem 

 wood cell, shown in fig. 31,^, shows the more primitive 

 type of wood cell, which has an oblique cross wall. This 

 type of wood cell is found in all the fossil trees, and all 

 the living plants except the flowering plants. The vessel 

 type, which is that in the big wood vessels of the flower- 

 ing plants, and has no cross wall, is seen in fig. 20, x. 



The similarity between the living cells and those of 

 the Coal Measure fossils is sufficiently illustrated to need 

 no further comment. This similarity is an extremely 

 helpful point when we come to an interpretation of the 

 fossils. In living plants we can study the physiology of 

 the various kinds of cells, and can deduce from experi- 

 ment exactly the part they play in the economy of the 

 whole plant. From a study of the tissues in any plant 

 structure we know what function it performed, and can 

 very often estimate the nature of the surrounding con- 

 ditions under which the plant was growing. To take a 

 single example, the palisade tissue, illustrated in fig. 25,^, 

 in living plants always contains green colouring matter, 

 and lies just below the epidermis, usually of leaves, but 

 sometimes also of green stems. These cells do most of 

 the starch manufacture for the plant, and are found best 

 developed when exposed to a good light. In very shady 

 places the leaves seldom have this type of cell. Now, 

 when cells just like these are found in fossils (as is illus- 

 trated in fig. 34), we can assume all the physiological 

 facts mentioned above, and rest assured that that leaf 

 was growing under normal conditions of light and was 

 actively engaged in starch-building when it was alive. 

 From the physiological standpoint the fossil leaf is 

 entirely the same as a normal living one. 



