OF THE LEAVES OF BRITISH GRASSES. 325 
Sometimes the bands of stereome under the lower epidermis broaden out laterally 
and may even become united with one another, thus forming a continuous band of 
sclerenchyma under the lower epidermis. This is seen, for example, in the maritime 
grasses Psamma arenaria (Pl. 36. fig. 12) and Agropyrum junceum (Pl. 36. fig. 13) and 
in some varieties of Festuca ovina (PI. 36. fig. 4). 
Occasionally we find supernumerary bands of stereome. Thus in Aira cespitosa 
(Pl. 37. fig. 20, and PI. 40. fig. 67, st.), Alopecurus geniculatus (Pl. 37. fig. 21), and a few 
other grasses we find bands on the lower side opposite the bottoms of the grooves, In 
these cases the leaf between the ribs is usually thin, and the extra bands are therefore 
necessary for support. Again, in Aira flexuosa (Pl. 36. fig. 6) the mechanical system is 
represented on the lower (outer) side of theleaf by alarge number of very small stereome- 
bands, arranged very close together without any reference to the vascular bundles. 
Then also in Glyceria aquatica (Pl. 38. fig. 37) we find, in addition to the girders to the 
vaseular bundles, numerous small bands under both upper and lower epidermis; these 
probably function in supporting the leaf, which is much weakened between the bundles 
by the development of the large air-lacune. 
The primary function of the mechanical tissue is, of course, that of support, and the 
arrangement of bands and girders found in grass-leaves is that best calculated to afford 
support to these long, linear, bilateral structures, as Haberlandt * has shown. 
The extreme abundance of mechanical tissue in some maritime grasses seems, however, 
to drive Raunkiær to seek for other functions for the stereome. Thus he speaks of the 
strengthening tissue of Psamma arenaria and Agropyrum junceum “serving as a 
protection against evaporation." He also suggests that the strengthening tissue may 
serve for water-storage. He makes mention also of a rule “that the strengthening 
tissue increases in massiveness and in firmness in proportion as the moisture of the soil 
decreases and the conditions for transpiration increase.” 3 
In the first place, I question very much whether this rule holds good in grass-leaves. 
The heath-grasses are perhaps even more specialized for xerophytic conditions than those 
of maritime sands, but here we certainly do not find any excessive development of 
mechanical tissue ; in Aira flexuosa (Pl. 36. fig. 6) indeed, the most highly specialized of 
the group, the amount of strengthening tissue is relatively very small. Then, again, in 
certain water-grasses, as Arundo Phragmites (Pl. 39. fig. 49), the stereome is strong and 
abundant. i 
Again, when we consider the strength of the lower epidermis of Agropyrum junceum 
and Psamma arenaria, the great cutinization and the flatness of the outer walls of its 
cells, and the entire absence of stomata from the lower side of the leaf, it would appear 
doubtful whether the presence of a subepidermal band of lignified cells would materially 
reduce the transpiration. ah 
It seems, therefore, to me most probable that the sole function of the stereome is that 
of support, and that we can explain its variations entirely from this point of MIT. 
The leaves of the heath-grasses are subulate and narrow ; their mechanical tissue is 
therefore comparatively small in amount, and, as is typically the case in cylindrical 
* Haberlandt (1896), pp. 156 &c. 
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