14 EVOLUTION OJ-^ PLANTS 



accumulation of doubtful and imperfectly studied 

 forms. 



While it is true that the great majority of the fossil 

 remains of plants are too imperfect to make possible a 

 satisfactory study of their liner structure, it happens 

 occasionally that the tissues are preserved with extraor- 

 dinary completeness, so that a microscopic study of 

 the cellular structure is possible, and in this way much 

 light has been thrown upon the real nature of many 

 fossil plants. A few types, like the Diatoms, have 

 silicified cell-walls which have remained unaffected by 

 the changes to which they have been subjected, and 

 have sometimes been preserved in immense quantities, 

 and so perfectly that even the species can be determined 

 without difficulty. Other forms, with calcified cell- 

 membranes, like the Coralline algse and Characese, have 

 also been preserved very perfectly. In the vascular 

 plants the preservation of the cell-structure is usually 

 due to the infiltration of silicious matter after the 

 death of the plant. These silicified tissues, such as 

 the familiar fossil woods, often show the cell-structure 

 with marvellous clearness, but unfortunately the more 

 perishable tissues are very seldom preserved in this 

 way, and these are usually of especial importance in 

 classification. Thus the flowers of the seed plants, and 

 the spore-bearing parts of the lower ones, are seldom 

 preserved in a recognizable state, and this makes a 

 careful study of such few forms as have survived, 

 doubly important, as these are the surest means of de- 

 ciding the relationships of these fossil plants to each 

 other and to their living descendants. At best, the 

 geological record is extremely fragmentary, and we 



