102 BOTANY. [CHAP. in. 



and stomata, for it will be remembered that leaves can 

 only perform their most important function, that of 

 assimilation, when exposed to light; hence it would be 

 a great advantage if leaves could pack themselves up 

 during the night when unable to work, and this point 

 has been reached by many Dicotyledons. The leaves of 

 many Dicotyledons are still rigid and incapable of pro- 

 tecting themselves by closing up at night or on dull days 

 when the light is insufficient to enable them to perform 

 their functions ; but the vast majority are moving in 

 this direction, the first indication of such a move being 

 an indication of toothing along the margin. From the 

 primitive phase of cut margin as illustrated by the 

 cherry or apple leaf, there is a sequence in the depth 

 of the indentations through the stage presented by the 

 ivy and the dandelion, until eventually we get the 

 cutting so deep that it reaches to the principal vein or 

 midrib of the leaf, and the portions of cut-up blade or 

 lamina are only attached to the midrib by the principal 

 vein of each portion of the leaf by a joint or articu- 

 lation that renders possible the movement of the 

 jointed portion called a leaflet; when this stage has 

 been reached the leaf is described as compound. There 

 are two types of compound leaf, the earliest being 

 illustrated by the dog-rose, and known as the pinnate 

 type, where several pairs of leaflets are arranged 

 at some distance apart along the midrib, the latter 

 being terminated by a single leaflet ; the leaves of 

 peas, beans, and laburnum belong to this type. The 

 second plan is where all the leaflets spring from the end 

 of the stalk like a fan, as in the horse-chesnut and lupin. 

 This last arrangement is brought about by the arrest or 



