266 



THE YOUNG 



NATUEALIST. 



the individual leaves on a twig you 

 meet with five leaves, and have twisted 

 twice round the stem before you come 

 to one exactly above the first. In the 

 privel and periwinkle, two leaves spring 

 from the same level on opposite sides 

 of the stem ; whilst in the mints, pinks, 

 and many other plants, these opposite 

 pairs cross at right angles, the third 

 pair being exactly above the first. In 

 the well-known woodruff, or the 

 familiar " cleavers of the hedges, the 

 leaves form a wheel-like circle around 

 the stem, and each whorl alternates 

 with the one immediately above or 

 below it. The object of these arrange- 

 ments being to expose as much as 

 possible of the surface of the leaves to 

 the sun and air. 



A perfectly- developed leaf is com- 

 posed of three parts — a sheath which 

 clasps the stem, a more or less evident 

 stalk (petiole) which supports the flat- 

 tened blade. Such a typical leaf can 

 be seen in the common field buttercup 

 and in many of the umbelliferae. More 

 generally only two parts are present, 

 as stalk and blade in beech and elm ; 

 or sheath and blade, as in grasses ; 

 frequently only the blade is developed, 

 as in stocks and wallflowers ; whilst 

 more rarely only the petiole is present, 

 as in a common greenhouse acacia 

 popularly know as the "bottle brush" 

 — a seedling of this plant shows the 

 gradual transition from fully-formed 

 leaves to these flattened leaf-like stalks, 



which have always their surfaces verti- 

 cal instead of horizontal to the plane 

 of the horizon. Such plants are com- 

 mon in Australia, and when the sun 

 shines through the foliage it gives to 

 our eyes a weird and bizarre appear- 

 ance to the landscape. Many plants 

 such as peas produce tendrils, which 

 are slender thread-like prolongations 

 of the midribs of the leaves : these 

 become exquisitely sensitive, and being 

 endowed with motion, seize hold of 

 surrounding objects and thus raise the 

 plant into the air, which its otherwise 

 feeble stem would fail to do. The 

 same function is performed by the 

 leaf-stalks of certain other plants, such 

 as the garden nasturtium, which tightly 

 clasp contiguous twigs or other avail- 

 able support. A curious appendage 

 of leaves (stipules) occur in various 

 plants : they are very conspicuous in 

 the pansy and garden pea ; in one 

 rather rare vetch (Lathyrus aphaca) 

 they entirely supplant the true leaves, 

 which become aborted into tendrils. 



Leaves may be arranged in two 

 classes — simple and compound. By 

 simple are meant those with only one 

 stalk and one blade, as in oak and 

 apple; whilst compound are those 

 which, although having only one at- 

 tachment to the stem, have numerous 

 leaflets jointed tj the common stalk, 

 as in ash or horse chestnut. The 

 ramification of the veins in leaves are 

 of even wider significance in the sys- 



