THE GROWTH OF THE SHOOT FROM THE BUD 57 



up either in the form of sugar, or it may be reconverted into 

 starch. 



If the stems of plants growing in a hedge are compared with 

 each other, great variety will be found. Some will be green 

 and slender, others will be yellowish-brown, and a third set may 

 have the power of twining. The green stems in all probability 

 belong to plants that are annuals, i.e. they grow up, bear leaves, 

 form flowers and fruit, then die down at the end of the season. 

 Such a plant has no great weight of leaves to bear, and therefore 

 can manage with a slender stem ; it has to make food as rapidly 

 as possible, and for this reason chlorophyll, which gives the 

 green colour, is necessary to it. Further, as the plant dies down 

 in the autumn the stem will not be exposed to the cold and damp 

 of winter, and therefore needs no protection. The case is different 

 with biennials. They do not produce seed until the second 

 season, and therefore have to face the winter cold. Underneath 

 the thin skin of the stem, a layer of cork is formed. This protects 

 the underlying tissues, and may be recognised by its yellowish 

 colour. Some plants are perennials. Their stems remain above 

 ground several years, and go on increasing in thickness. These 

 plants often have branched stems, because it is the only way 

 in which they can get enough light for their leaves. A branch 

 always arises, as we have 5 



already seen, in the angle 

 which the leaf makes with 

 the stem. This is shown 

 very clearly in the accom- 

 panying illustration, which 

 is magnified about fifty 

 times. It also shows the 

 layer of cork developed at 

 the base of the leaf-stalk 



in the autumn. As SOOn 



. , . . r 



as this is formed the cell 



sap from the stem cannot get to the leaf, which, deprived of 



nourishment, dies. 



Plants growing in hedges often have twining stems. The 

 Black Bryony is one of the commonest. At the bottom of a 



c 



FlG> 3 1. Section through stem and axillary buds. 

 s. stem ; /, leaf; b, bud ; r, layer of cork. 



