THE PHYLOGENY OF GRASSES 1 



WILLIAM H. LAMB 



Forest Service, Washington, D. C. 



The purpose of this paper is to give a brief discussion of the 

 origin of grasses, and also to consider the essential differences 

 between the tribes of grasses and their probable derivation from 

 a common ancestral type. Grasses have come from the lilies 

 or from lily-like plants by a reduction in the number of the parts 

 of the flower. From the fact that grasses differ from the lilies 

 in just those characteristics which make them more likely to 

 survive the attacks of voracious animals, it may be that the 

 stress of being fed upon has been an important factor in the 

 evolution of the grasses. 



In order that we may understand the changes that have taken 

 place in. the lilies to produce the grasses, let us consider the 

 structure of a typical lily. Perhaps the Easter lily is the most 

 familiar example. Here we find apparently six white petals but 

 we know that the outer three are sepals. We notice also that 

 there are six stamens in two whorls of three each, and a three 

 lobed or tricarpellary pistil. The first step was a reduction in 

 the lobes of the pistil by causing them to become smaller, func- 

 tionless, and finally to disappear. Along with this reduction in 

 the lobes of the pistil came a greater and greater tendency to 

 take on the grassy habit of growth. A modification of the peri- 

 anth also took place. As we pass through the rushes the peri- 

 anth becomes more " chaffy" and at last is reduced to mere 

 bristles, although in some of the rushes we find that one whorl 

 has disappeared and that the pistil no longer has three lobes. 

 These modifications in addition to a grassy appearance seem to 

 indicate that these rushes (the Eriocaulaceae) are intermediate 

 between the lilies and the grasses. The first grasses were prob- 

 ably all species of the same genus — a primitive group, probably 



1 Published by permission of the Secretary of Agriculture. 



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