CYTOGENETICS AND EVOLUTION OF THE GRASS FAMILY I7I 



anatomy (distribution of chlorophyll-bearing tissue) and particularly the 

 inflorescence and spikelets. The predominant trend, expressed in every genus 

 of the Panicoideae, is reduction of the fertile florets to one per spikelet. 

 In the Panicoideae this affected chiefly the lower florets of the spikelet, 

 while in the Chloridoid-Eragrostoid line the upper florets were more strongly 

 affected. In addition, various branches of both lines, but particularly the 

 Panicoideae, have developed a tremendous diversity of specialized adaptations 

 for seed dispersal (fig. 1 ) . Among the most striking examples are the bristly 

 involucres of the elephant grasses (Pennisetum) , which evolved further 

 through fusion of bristles to form the involucre of the sand burs (Cenchms) ; 

 the long-awned, pointed callused fruits of tanglehead, or Pili grass {Heter- 

 opogon), which simulate those of Stipa, discussed below; the upper leaf 

 sheaths of Job's tears {Coix), which have become inflated, hardened, and 

 polished so that they resemble beads; and the enlarged, indurated rachises, 

 or cobs, of the maize tribe (Maydeae). 



The third dominant line, that of the Festucoideae, are the principal grasses 

 of temperate climates. They became specialized largely by reduction in the 

 leaf epidermis, caryopsis, embryo, and seedling. The specialization of the 

 inflorescence and spikelets lagged behind that of the Panicoideae, although 

 it reached a high level in several genera of the Agrostideae and Phalarideae. 

 The large chromosomes of the Festucoideae are probably a later specializa- 

 tion, as suggested by Avdulov, since they do not occur in any grasses which 

 are primitive in morphological characteristics. Avdulov's suggestion that 

 large chromosome size evolved as an adaptation to winter cold is, however, 

 hard to justify on the basis of any known facts of chromosome chemistry 

 or cellular physiology. It is, however, an intriguing speculation, particularly 

 since it is paralleled to some extent in other families. In the Commelinaceae, 

 Liliales, Leguminosae, and Polemoniales the largest chromosomes are found 

 in species inhabiting temperate climates; although, of course, many families 

 with consistently small chromosomes (Cyperaceae, Juncaceae, Rosaceae) 

 are dominant in these same temperate regions. The relationship between 

 chromosome size, cellular metabolism, and growth is a subject which cer- 

 tainly deserves attention and about which new discoveries of considerable 

 evolutionary significance might be made. 



A less successful line of radiation, the bamboos, became dominant only in 

 the moist forests of the tropics. Some of its members have developed extreme 

 vegetative specializations, such as the habit of climbing over other vege- 

 tation by means of elaborate systems of spines or thorns developed on the 

 stems. As mentioned above, many species of bamboos have very primitive 

 reproductive structures, but others are highly specialized in this respect. 



Three additional lines of grass evolution were highly successful in even 

 more limited ways. The Arundineae, or reed grasses, which retained the 

 apparently primitive chromosomal condition of a basic number of x — 6 or 12, 



