74 ANATOMY OF LEAVES. 



observed in a dead leaf for some time exposed to the weather. In ihis 

 will be seen a rib running midway through it and numerous fibres 

 branching from it towards the margin. The skin may be removed by 

 boiling it slightly and rubbing out the cellular texture, leaving the 

 different vessels or the vascular system entire. They then present 

 (he appearance of veins and arteries, being tubular, etc. This will 

 now be more apparent by immersing it in a colored fluid which will 

 penetrate the fibres. The skin or cuticle which covers and protects 

 the vascular system, is sometimes covered with down or hairy glands 

 for security against variations of the weather, and sometimes with 

 a clear varnish which protects the leaf against too much moisture. 

 This is common in parts where there is much rain. The leaf is said 

 to be regose when the cellular tissue abounds and the upper surface 

 swells while the under surface becomes depressed, and cancellated 

 when there is little cellular tissue and the vascular system presents 

 the appearance of net-work. The rib proceeds from the upper end or 

 the petiole of the leaf-stalk through the middle of the disc ; and the 

 branches proceeding from this are called riblets. Grasses have the 

 simplest form of leaf. Beside the mid-rib, some leaves have two, 

 others one at each side and are called three-nerved leaves. The 

 grass leaf has one, the ivy leaf two and the grape leaf three. Simple, 

 leaves have one, the main rib with its branches ; and the branches 

 of the compound leaves divide the disc into many forms. 



The, physiology of leaves is of the greatest importance to plants, and, 

 indeed, to the whole animal world. The upper part of the leaf per- 

 forms the respiratory functions : it is of a deeper green than the 

 under part ; but when the upper part is placed on the surface of 

 water, this color diminishes and the leaf withers. The under surface, 

 however, with like circumstances, will preserve the leaf fresh for 

 many days. Plants almost invariably present their upper surface to 

 the light, at which time it gives off oxygen by decomposing the car- 

 bonic acid which the plant receives through the roots and also that 

 received from the air through the leaves, the carbon remaining and 

 ultimately forming starch, gums, sugar, and other solid parts. But in 

 the dark, instead of oxygen, carbonic acid is given out, so that during 

 the day leaves yield oxygen to the air and withdraw carbonic acid, 

 while during the night they give out carbonic acid, but no oxygen. 

 The labor thus performed during the day is generally in proportion to 

 the intensity of light and the circumstances of their situation. Some 

 plants close there leaves at a certain period of the day and open them 

 at another, as with the sensitive plant ; and most of them shrink, fold 

 their leaves or corollas and cease to act after the disappearance of 

 the sun, but resume their task on the apearance of light in the morn- 

 ing. This has been termed the sleep of plants, which is doubtless as 

 necessary to them as sleep to animals. Other than solar light will 



