130 HEREDITY AND EVOLUTION IN PLANTS 



arrangement of plant groups as representing the general 

 course of their evolution (Table I) : 



From what has already been said, however, it should 

 be understood that such a table represents, not the line 

 of evolutionary advance, but the paths travelled by plants 

 in the course of their development. For example, it im- 

 plies that dicotyledons were derived from monocotyledons, 

 pteridophytes from bryophytes — ^hypotheses which, from 

 other trustworthy evidence, as stated above, now seem 

 untenable. 



Table I . — Sequence of Plant Groups, Based on the 

 Morphology of Living Forms 



Thallophytes J Algae — having chlorophyll, 



(no archegonia) \ Fungi — no chlorophyll. 



Archegoniates 

 (archegonia, but no seeds) 



Spermatophytes 

 (seeds) 



Bryophytes — no vascular system. 

 Pteridophytes 1 



Calamophytes \ vascular system. 

 Lepidophytes J 



Gymnosperms — no closed ovary. 

 Angiosperms — closed ovary (pistil). 



Monocotyledons — one-seed leaf. 



Dicotyledons — two-seed leaves. 



Again, the table suggests that Angiosperms were de- 

 rived from Gymnosperms, and therefore appeared late 

 in the history of plant life; but the study of fossil plants 

 shows that they appeared in the geological past, and w^ere 

 dominant in the Tertiary period, as now. We are led, 

 therefore to proceed with caution in drawing inferences 

 based only upon a comparative study of the structure of 

 forms now living. 



104. Consequences of an Amphibious Habit of Life. — 

 The life history of the fern affords a concrete illustration 



