EVOLUTIONARY HISTORY OF PLANTS 255 
familiar objects in woods, fields, and gardens, the beginner 
in botany is apt to feel that they are in all ways the prin- 
cipal plants. This is a serious mistake. Jn the classifica- 
tion of plants in Sect. 261 something of scientific accuracy 
has been sacrificed for the sake of simplicity. Really, 
according to the highest living authority on the subject, 
Professor Adolf Engler, seed-plants form only one out 
of thirteen branches of the vegetable kingdom. In their 
inconceivable numbers spore-plants far surpass seed-plants. 
While a little scientific knowledge of plants has existed 
for more than two thousand years, most of our knowledge 
of such minute plants as bacteria dates back much less than 
half a century, to the time when the use of sufficiently 
powerful microscopes first made their study possible. 
The importance of cryptogamous plants to human life 
and work is too great to be explained in a paragraph, a 
chapter, or even in a single book. From the clover field 
to the baker’s kneading trough, from the cheese factory to 
the hospital for consumptives, modern life is full of prac- 
tical illustrations of the bearings of cryptogamic botany 
on agriculture, manufactures, and medicine. 
331. The Earliest Plants.— The question, What were 
the first forms of plant life that existed on the earth’s 
surface? cannot be precisely answered. The earth in its 
solid condition probably remained for a long period as an 
intensely heated mass, destitute of any trace of life. Ages 
later, but many hundreds of centuries ago, the tempera- 
ture of the globe became sufficiently lowered to admit of 
the existence of plant life. These earliest plants were 
undoubtedly of the simplest form and may very probably 
have been one-celled aquatic alge. Such plants, however, 
would have been unlikely to leave recognizable remains 
