286 



HUTCHINSON'S POPULAR BOTANY 



the procumbent position of the lace-like leaves, which form a rosette round 

 the short mud-embedded stem. They remind one, as Kerner aptly says, of 

 autumn leaves which have fallen into water and lost all their parenchyma 

 through maceration, the skeletons alone remaining. It may be added that 

 a few of the Seaweeds (e.g. Agarum gmelini and Thallasiophyllum clathrus) 



offer the same peculi- 

 arity as Ouvirandra, 

 their fronds being per- 

 forated in a very beauti- 

 ful manner. 



The existence of 

 leaf-holes in certain 

 land-plants is also to be 

 noted. Such perfora- 

 tions are confined to the 

 large upper leaves of 

 tropical plants like the 

 Aroids (Monstera deli- 

 ciosa, etc., fig. 348), 

 which, but for this pro- 

 vision, would entirely 

 exclude the sun from 

 the lower leaves, and 

 thus impair the activity 

 of the green tissues. The 

 deep incisions and clefts 

 which give such beauty 

 of outline to palmati- 

 sect and pinnatisect 

 leaves evidently sub- 

 serve a similar purpose ; 

 while the disposition of 

 the leaves on the stem, 

 and of the leaflets on the 

 petiole, has definite rela- 

 tion to the same impor- 

 tant end. 



It is highly probable, 



also, that the laciniated (fringed) forms of specially large leaves bear the 

 same relation to the wind that the thread-like forms of submerged leaves 

 do to water that is, they present no large unbroken surfaces to the 

 varying currents of air, and thus escape rupture during heavy storms. In 

 many cases tearing is prevented by a strengthening of the epidermal cells, 

 particularly at the edges of the leaves, where of course the strain is 



Photo 



\E. Step. 



FIG. 349. REEDMACE (Typha latifolia). 



Commonly confused with the Bulrush (Scirpus laatstris). Kote the spiral twist 

 of all the long strap-shaped leaves, so that the whole surface is never pre- 

 sented to the wind. 



