PLANT MORPHOLOGY 



Sphenopsida, and Pteropsida. The first three correspond to established 

 classes of pteridophytes under the older classification, while the Pterop- 

 sida include the ferns (Filicinae), gymnosperms, and angiosperms. 



The presence of leaf gaps in the vascular cylinder is thought to indicate 

 a closer relationship between the ferns and seed plants than exists 

 between the ferns and other pteridophytes. However, the basis used in 

 distinguishing the ferns from the gymnosperms and angiosperms, when 

 the three are grouped together as Pteropsida, is the same as when the 

 ferns are placed in the Pteridophyta and the other two groups in the 

 Spermatophyta. Furthermore, if the existing classes of pteridophytes 

 represent collateral lines of descent from the psilophytes of the Devonian, 

 a view widely accepted, their relationship to one another can better be 

 expressed by including them in a division of their own. Certainly no 

 greater degree of relationship is expressed by placing them in a division 

 that also includes the gymnosperms and angiosperms. 



The t"svo different schemes of classifying the embryophytes are as 

 follows: 



B. Embryophyta 



I. 



I. Bryophyta 



1. Hepaticae 



2. Musci 



II. Pteridophyta 



1. Psilophytinae 



2. Lycopodiinae 



3. Equisetinae 



4. Filicinae 

 III. Spermatophyta 



1. Gymnospermae 



2. Angiospermae 



Bryophyta 



1. Hepaticae 



2. Musci 



II. Tracheophyta 



1 . Psilopsida 



2. Lycopsida 



3. Sphenopsida 



4. Pteropsida 



a. Filicinae 



b. Gymnospermae 



c. Angiospermae 



PLANT LIFE OF THE PAST' 



The plants of today are the modified descendants of other plants that 

 have lived on the earth throughout the course of geologic history. They 

 are the products of a process of evolution that has been in operation since 

 life first began. Our knowledge of the plants of the past has come from a 

 study of fossil remains found embedded in the layers of rock that form the 

 earth's crust. These remains constitute a direct record of the changes 

 undergone by plants down through the ages. This record, incomplete 

 as it is, helps us to follow the course of evolution and to understand the 

 relationships that occur among the various existing plant groups. 



It is not known how or when life arose on the earth. It is not even 



known in what form it arose, although much evidence indicates that the 



first living things were extremely simple and from them forms more and 



' This subject is presented in much greater detail in Arthur W. Haupt, An Introduc- 

 tion to Botany, 2d ed., Chap. XX, New York, 1946. 



