Miscellaneous. 



192 



In very shady places, as for example along the streams in most up-country 

 jungles, the leaves can often be seen beautifully arranged in what is sometimes 

 called a leaf-mosaic (Plate II) spread out so as to occupy every bit of space without 

 overlapping. In many cases they will also be seen to be unequal (Plate II), usually 

 those on the lower side of the stem larger than those on the upper side, but not 

 always. The reason of this phenomenon has not yet been satisfactorily made out, 

 but it is very common. 



Leaves may have stalks or petioles, or have none (i.e., be sessile), and these 

 stalks, by being of greater or less length as needed, help very much in the construc- 

 tion of leaf mosaics. 



The leaf itself is a thin green expanded organ in most cases, and is in a 

 sense the most important part of the plant, as it prepares, irom the materials 

 coming to it from the roots (water and many substances dissolved in it) and 

 from what it gets from the air, the actual food upon which the growing parts 

 of the plant feed. The great bulk of a plant is made up of material derived 

 from the almost infinitesimal amount of carbon dioxide gas in the air (the gas given 

 out by animals and plants in breathing) by means of the leaves, and of water taken 

 from the soil. The plant, in the enormous majority of cases, is absolutely unable to 

 get water from rain by means of its leaves, or to get carbon from the soil even if the 

 latter contain a good deal of it. 



In order to absorb carbon freely from the air, the leaf in places where it is 

 not too dry for such construction is very thin and spread out flat by means of 

 stronger veins or nerves running through the green tissue. These veins run down to 

 the base of the leaf and enter the stem. They are the channels by which the water 

 from the root enters and spreads out in the leaves, and are very much ramified 

 through the latter, so that every part shall easily get its water. 



If the leaves are to be exposed to much sunshine, the very thin and delicate 

 structure seen in so many leaves will not suit, and they are more leathery, as is seen 

 for example in jak leaves. 



Leaves when young can still move to some extent, and they in general take 

 up what it is often called their fixed light position, arranging themselves so as to be 

 at right angles to the brightest diffused light (not, as a rule, direct sunlight) falling 

 upon them. In other words, they tend to be horizontal, but if, for example, the 

 plant is placed in a window they Avill all grow to face the light, and stand at a 

 considerable angle with the vertical. 



What we have said refers to ordinary leaves, which are what is called 

 dorsivenlral, having a dorsal or lower surface and a ventral or upper, which differ 

 in internal structure and usually in external appearance. Some plants however have 

 their leaves the same on both sides, and these isobilateral leaves stand with their 

 points or edges to the light, as may be seen in the Gladiolus, &c. In the Acacias and 

 some kinds of Eucalyptus, e.g., the blue gum, the leaf is replaced by an organ standing 

 edgewise to the light and called a phyllode, which is really the flattened leaf stalk. 

 Often intermediate stages (Plate II) can be found on the plant, the phyllode not 

 being quite so large, and the leaf blade not having entirely disappeared. 



Many leaves, especially in the plants of the family Leguminosae, to which 

 peas and beans belong, sleep at night or in a hot sun. The sensitive plant, so common 

 in the low country, also sleeps when touched, but few are so sensitive as this. Most 

 of them, however, bend down their leaflets in various ways at night (Plate II), so as to 

 turn their edges to the sky instead of their flat surfaces. The common species of 

 Oxalis also show this very well. 



(To be continued.) 



