340 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



together lengthwise; again after sunset they widen and become flat or fluted. This 

 process may be repeated twice on a summer's day within twenty-four hours, if a 

 storm intervenes at mid-day and is followed hy a sunny afternoon. How much 

 this depends upon the conditions of humidity of the air, is demonstrated by the fact 

 that such grasses, when grown in pots, can be easily made to open and close their 

 leaves by alternately sprinkling them with water and placing in damp air, and then 

 for a short time exposing them to dry air. The leaf -folding in various species 

 of Sesleria is exceedingly quick and also very interesting. The species of this 

 genus grow principalljr on the Alps, Carpathians, and Balkans. They always 

 grow together and often cover wide stretches of hilly and elevated districts with 

 thick grassy turf. One species (Sesleria coerulea) is distributed over Northern 

 Europe in Finland, Sweden, and England. The closing of the leaves of these moor- 

 grasses reminds one strongly of the Venus Fly-trap (Dioncea onuscij^ula), which has 

 already been fully described. It is indeed an actual shutting together of the two 

 halves of the leaf. As in the leaf of the "Fly-trap", the midrib of the leaf of the 

 Sesleria remains in its original position unaltered ; also the two halves of the leaf do 

 not come flatly in contact, but rise up obliquely so as to leave between them a deep, 

 narrow, groove-like cavity, widest at its lowest part (see fig. 85^). While the open 

 leaf turns its upper surface, rich in stomata, towards the sky, the two raised halves 

 of the folded leaf are parallel with the incident sunbeams, and the folded leaf of the 

 moor-grass may then be compared to the equitant leaf of an iris. In the cavity 

 produced by the closing up of the leaf are the stomata, however, and thus the green 

 tissue next them is excellently protected from the sun's rays as well as from the 

 direct action of the wind. The epidermis of the lower surface, which is exposed on 

 the folded leaf to all the agencies which excite transpiration, possesses no stomata, 

 but is provided with a thick cuticle. 



A leaf-folding similar to that of Sesleria, along the midrib, has been observed in 

 the leaves of Avena planiculmis, which grows in sunny fields on the Sudetics and 

 Carpathians. It also occurs in A vena conipressa, and many others related to these 

 species. The folding or closing of the leaves in the large section of fescue-grasses 

 (Festuca) is carried on somewhat differently. In Sesleria, the opened upper sur- 

 face of the leaf forms only a single shallow groove, and the folding only occurs at 

 the midrib; but on the upper side of the fescue-grass leaf several parallel grooves 

 are to be seen, and the green tissue is divided up by these grooves into several pro- 

 jecting ridges, exhibiting a very remarkable structure. In each ridge can be dis- 

 tinguished the base which forms a part of the under side of the whole leaf; then 

 the apex opposite the base, belonging to the upper surface of the entire leaf; and 

 finally, the two side portions forming the sloping sides of the grooves which run 

 between the ridges (see figs. 87 and 88). 



The greater part of each ridge consists of green tissue. The stomata on the 

 ridge only open on the sloping sides facing the grooves. Neither the crests of the 

 ridges nor the lower surface of the leaf exhibit a single stomate. The apex is without 

 chlorophyll, and almost always has a border of elongated cells with strong elastic 



