CYTOGENETICS AND EVOLUTION OF THE GRASS FAMILY 1 73 



remained primitive in some other respects, such as the lack of reduction in 

 the spikelets, and preserve a considerable diversity in epidermal and ana- 

 tomical structures. Isolated genera of this group, such as the common reed 

 (Phragmites) , though probably very old, are still successful throughout 

 the world, while others such as pampas grass (Cortaderia) have developed 

 chiefly in the Southern Hemisphere. Still other genera of Arundineae (Am- 

 pelodesmos, Lamprothyrsus) are relics, surviving in only a restricted part 

 of the world, although they are successful and common in their areas of 

 distribution. 



The Oryzoid line, of which the best-known member is cultivated rice, 

 has developed some of the most successful grasses of wet places. One peculi- 

 arity of this line is that many of its species have preserved the primitive 

 condition of six stamens per floret, a condition existing elsewhere in the 

 family only in the bamboos and a few other isolated genera. The most char- 

 acteristic morphological feature developed in this line is the strong flattening 

 of the spikelets and pronounced reduction in both glumes. 



The Stipoid line is probably an offshoot of the danthonoid-arundinoid 

 complex which became highly specialized in fruit structure. The develop- 

 ment of a thick, tough fertile scale, or lemma, with overlapping margins 

 was followed by further specialization in two directions. In the genus Stipa, 

 or needle grasses, the fruit became much elongated, and the rachilla, or 

 spikelet axis, immediately below it became modified into a sharp-pointed 

 callus. At the same time, the awn at the fruit apex became much elongated 

 and twisted in such a way that it winds and unwinds with changes in mois- 

 ture. In this way the fruit has been converted into a drill, which effectively 

 buries the seeds in the ground and increases their chances of successful germi- 

 nation under low moisture conditions. A further specialization, the develop- 

 ment of hairs or plumes on the awn, is an aid to seed dispersal by wind, 

 and particularly by animals, through clinging to their hair. In the other 

 genera of this line, such as the rice grasses (Oryzopsis) and two principally 

 South American genera (Nassella, Piptochaetium) , the fruit has become 

 rounded in shape, the lemma possesses a hard, shining surface, and the awn 

 is relatively weak. At first sight, these fruits would appear to be very poorly 

 equipped for wide dispersal, an appearance which is not borne out by the 

 extensive distribution of many species of Oryzopsis. A suggested explanation 

 for their dispersal is that these hard, shining fruits are particularly resistant 

 to the digestive juices of animals, and so are carried long distances in the 

 intestines of mammals and the crops of birds. At any rate, both Stipa and 

 Oryzopsis have become among the most successful grasses of cold steppe 

 regions. 



In addition to these seven successful lines, the grasses contain many 

 peculiar monotypic, ditypic, or small genera with very uncertain affinities. 

 Among the most striking examples are Nardm and Lygaeum in Europe, 



