64: EASY LESSONS IN VEGETABLE BIOLOGY. 



leaves are alternate or scattered. Irregular as the 

 latter mode may appear in different plants, observa- 

 tion shows that it is tolerably uniform in each species. 

 f If a spiral line (or thread) is drawn round 

 the stem connecting the points of attach- 

 ment of the leaves, and these are marked 

 on the spiral, it is found that in any par- 

 ticular species there is a definite number 

 of leaves on any given number of turns 

 made by the spiral round the stem. In 

 the peach and plum the cycle made by 

 the leaves directly above each other em- 

 braces five leaves, and the spiral goes 

 twice round the branch. This is expressed 

 by the formula -J-- I n the alder three 

 leaves form the cycle, and the spiral has 

 but a single turn on the stem. This is 

 represented by the fraction . 



Covering-leaves are so called because 

 they cover or protect other parts, as the 

 scales of buds, and bracts, or leaves in 

 axils of which flowers are placed. The 

 leaf-stalk is called a petiole. When it is 

 absent the leaf is said to be sessile. At 

 Mg ' 28 ' the base of the petiole flat leaf-like ap- 

 pendages are often found, called stipules. 



Leaves are said to be sini/ple when the blade is 

 composed of one piece, however irregular may be its 



