14 EVOLUTION OF 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 finer 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 alge and Characex, 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 
