No. 497] ON XEBOPHTT1C ADAPTATIONS SIS 



epidermis the stomata are often placed in rows at the 

 bottom of a groove. These grooves are very Blighl in 

 Nolina sp.f from Mexico (Fig. 15), Dasylerion wheeleri 

 and T). texanum. They are deeper in J). e/IaHco/Jti/lhnn 

 (Fig. 16) and Hesperaloe parvijlora* They are very 

 deep and guarded with teeth in XoHna tv.nnw* (Figs. 17 

 and 18) and Nolina sp.t from West Texas. The forma- 

 tion of grooves is directly associated with the bracing of 

 the epidermis with beams of mechanical tissue (Fig. 20) 

 along the ridges between the grooves. In Xolina sp.T 

 from Mexico (Fig. 15), Dasylerion wheeleri (Fig. 24), 

 D. texanum and Hesperaloe parviflora (Fig. 23) these 

 beams are free in the interior of the leaf, but in I). </hni< <> 

 pliyllum and Nolina texana (Fig. 20) the beams of one 

 side usually meet and fuse with those of the other, form- 

 ing a rigid support for the epidermis that prevents any 

 dorso-ventral shrinkage of the leaf on drying. The only 

 shrinkage that does occur closes the grooves and thus is 

 a further check to transpiration. Another characteristic 

 associated with the presence of these longitudinal trans- 

 piration grooves is a peculiar modification of the air 

 passages in the assimilation tissue, which reaches its 

 greatest development in the Nolinas. In the forms with- 

 out grooves there is an irregular air space below the 

 stoma, from which lead minute air spaces in all direc- 

 tions. With the development of grooves is associated a 

 more effective arrangement of these air spaces, so that 

 rapid respiration of the deeper tissue is effected without 

 the devotion of much of the volume of the leaf to air 

 space. If we imagine the groove represented in Fig. 17 

 in tangential section, as being placed horizontally with 

 the opening upward, the assimilation tissue in the form of 

 lamella? one cell thick is suspended from the inner surface 

 of the groove. Air diffusing in through the stomata 

 enters the narrow spaces between the lamella? and almost 

 immediately penetrates to the center of the leaf. If we 



2 Bray, '03. 

 'Bray, '03. 



