136 LEAVES AND BUDS. 



helow it by one-half the circumference of the stem. The next arrange- 

 ment (|) is seen in young shoots of Hazel, where the leaves are in 

 three rows. Alternate leaves obviously form a spiral, the nature of 

 which may be easily determined by tying a piece of cotton to the base 

 of a leaf, and carrying it in order to the bases of the leaves above, 

 twisting it around each one. Suppose, for instance, we do this in the 

 case of a young shoot of the Hazel ; we find that each leaf is one-third 

 round the stem as compared with the one below it, and consequently 

 the fourth leaf is vertically above the first, being the starting-leaf of a 

 second turn of the spiral. 



One of the commonest spiral or alternate arrangements is the f , seen 

 in Wallflower, Oak, and many other plants ; here the sixth leaf is 

 above the first and the spiral goes twice round the stem in passing 

 from the first leaf to the sixth. Another fairly common spiral is the 

 f , seen in Daisy, Plantains, etc. Do you notice how these commoner 

 spiral arrangements are related to each other ? 



When leaves are arranged in pairs, the two leaves being on opposite 

 sides of the " node," each pair is usually set at right angles to the pair 

 above and the pair below. That is, the leaves are in four rows, the 

 pairs being crossed. This decussate arrangement is very common, e.g. 

 Nettles, Labiates, Sunflower, Sycamore, Ash, Lilac, Horse Chestnut, 

 Campions, Stitchworts, Chickweeds. Comparatively few plants have 

 three or more leaves in a whorl or circle at each "node" ; this is seen 

 in Mare's-tail (Hippuris), Canadian Waterweed (Elodea), Juniper, 

 Heaths. The leaves are apparently in whorls in Goosegrass and Bed- 

 straws, but only two of the organs in each circle are true leaves (how 

 can you tell this ?). 



17O. Leaf-Mosaics. Instead of giving further details 

 here about the various modes of arrangement of leaves on the 

 stem, we will call attention to the reasons why leaves have 

 such varied arrangements and shapes. It is not always easy 

 to explain the different forms of leaves in different plants, but 

 some general principles can be inferred by remembering the 

 functions carried on by leaves while studying their form and 

 structure. The main fact to remember is that leaves re- 

 quire to catch as much sunlight as possible, especially in a 

 country like ours, where the number of hours of sunshine is 

 rather scanty. The south coast of England only gets about 

 1,600 hours of sunshine annually, and in the rest of Britain 

 the number is usually far below this. 



A leaf which receives very little light or no light at all 

 during its development grows feebly, and cannot, of course, 

 carry on carbon-assimilation. Moreover, when a leaf is 

 shaded by another leaf being placed between it and the light, 

 very little assimilation occurs. Hence we find that in many 



