INTRODUCTION 



23 



light having checked the formation of chlorophyll. 

 Young shoots, especially, require protection from the 

 action of light, which would otherwise check the forma- 

 tion of chlorophyll. This is afforded in several ways, 

 of which the principal are — by the 

 thick and often glossy epidermis ; by 

 the position of the leaf ; by the struc- 

 ture of the leaf; by the arrange- 

 ment of the chlorophyll granules ; by 

 change in their form ; by hairs, which 

 often fall off when the leaf is fully 

 developed ; by stipules — either those 

 of the same leaf, as in the Hop 

 [Humulus), or of an older leaf, as 

 in the Pea. The latter safeguards 

 are useful also in other ways ; for 

 instance, as a protection against cold, 

 against insects, etc. 



Again, the presence of red colour fiq. lo.— shoot of Black 



1 , 1 Poplar (PomUus niara). 



IS very common in young shoots, when 

 it perhaps serves to protect the chlorophyll in the 

 delicate tissues from too much light ; and this seems to 

 have been taken advantage of and developed in the 

 corolla and other parts of the flower, as an attraction 

 to insects. 



In ordinary parlance, when we speak of a leaf we 

 think only of the leaf blade. The leaf blade is only 

 one part of the leaf, which, if complete, may be 

 regarded as consisting of four parts — the blade, the 

 stalk, the stipules, and the base. One or more of these 

 parts, however, may be wanting. In some cases, for 

 instance, the leaf is sessile ; in others the blade is absent. 



Stipules are the two small leaflets which often occur 

 just above the base of the leaf, as, for instance, in the 

 Violet, Pea, Eose, etc. Linnseus gave them the name 

 from stipula, a little straw. This term happily 

 expresses their appearance in the case of the Beech. 

 In this species they drop early, and in spring make 

 the ground under Beeches almost brown. In this case, 



