318 MR. L. LEWTON-BRAIN ON THE ANATOMY 
Part L—GENERAL ANATOMY AND HISTOLOGY. 
GENERAL ANATOMY. 
The form of the grass-leaf as seen in transverse section is extremely variable. We 
may recognize four main types, not one of which is sharply marked off from the others, 
but all are connected by numerous transitional forms. 
1. Leaves in which the upper surface is either flat or practically so. Examples of 
leaves with a perfectly flat upper surface are most species of Poa, Glyceria (PI. 38. 
figs. 37, 39), Avena pratensis (Pl. 87. fig. 28), Triodia decumbens (Pl. 38. fig. 34), &e. 
Among those in which the upperside shows ribs that are practically obsolete may be 
mentioned Arrhenatherum avenaceum, Phleum pratense (Pl. 39. fig. 46), Brachypodium 
sylvaticum, different species of Bromus, Melica uniflora, and M. nutans. 
This type is said by Raunkiær to be “ only found in marsh- and wood-grasses, and in 
certain ‘mark’ and meadow-grasses.” This remark is true so far that, as will be seen 
later, most (though not all) wood and aquatic grasses possess this type of leaf; but 
included in this group we have also not only many meadow-grasses, but several grasses 
characteristic of waste places as well, e. g. Bromus sterilis (Pl. 37. fig. 27), Agropyrum 
repens, Setaria viridis (Pl. 38. fig. 36), Hordeum murinum, and others; heath-grasses, 
such as Zriodia decumbens (Pl. 38. fig. 34); maritime grasses, such as Poa bulbosa 
(PL 39. fig. 47) and Cynosurus echinatus (Pl. 39. fig. 51); and even alpine grasses, 
such as Phleum alpinum (Pl. 39. fig. 52). In fact all the biological groups have their 
representatives here. This point alone shows the extreme difficulty of making any 
generalizations on this subject. 
The stomata in this type of leaf are usually distributed on both surfaces. 
2. Leaves in which the upper surface is marked by distinet, though not very high, ribs. 
As examples of grasses with this type of leaf may be mentioned Loliwm perenne, 
Alopecurus pratensis (Pl. 37. fig. 18), and Festuca pratensis among meadow-grasses ; 
Festuca Myuros, Poa rigida (Pl. 37. fig. 23), and Alopecurus agrestis among vagabond- 
grasses; Alopecurus geniculatus (Pl. 3. fig. 21) among aquatic grasses ; Phleum arenarium 
and Poa loliacea among maritime grasses; and Agrostis canina (Pl. 37. fig. 19) among 
heath-grasses. 
This type of leaf is of a slightly more xerophytic character than the last one. Thus 
we get included here only one aquatic grass and no wood-grasses. The stomata are 
usually most abundant on the flanks of the ribs, and are thus more or less sheltered. 
Most of the grasses belonging to this series are meadow and wayside plants; they 
are exposed at times to fairly’ strong transpiration, but, on the whole, live under 
moderately favourable conditions as regards water-supply. The great majority of the 
two largest groups of grasses—from meadow and from waste places—possess leaves 
either of this type or of the preceding one, or are intermediate between the two. It is 
impossible to draw any hard-and-fast line between the two forms of leaf, the number of 
intermediate forms being even greater than that of the typical ones. 
