THE ANATOMY OF THE SHOOT 



73 



to indicate a genetic characteristic, variations often occur; and, 

 in a single axis, two or more types may develop. In Helianthus, 

 whorled, opposite, and alternate arrangements may develop on 

 one stem; and similar combination phyllotaxies occur in Linum, 

 Cannabis, and many other plants. Regardless of the system of 

 phyllotaxy, there is an adjustment of leaves in relation to light so 

 that the resultant leaf pattern or mosaic exposes a maximum 

 amount of leaf surface to it. 



\^ 



Fig. vj. Types of phyllotaxy. A, B, C, various alternate arrangements; D, opposite 

 arrangement. The upper figure in each case shows the phyllotaxy as seen from above. (From 

 Smith et al., Textbook of General Botany.^ 



Gross Characteristics. — A representative vegetative leaf com- 

 monly consists of an expanded blade or lamina and a stalk or petiole. 

 The latter is variable in length and in the sharpness with which it 

 is delimited from the blade. Some leaves are non-petiolate and ses- 

 sile. A special type occurs in the grass leaf, in which a basal sheath 

 surrounds the axis; and, at its upper limit, there may be a ligule 

 or collar-like membrane which clasps the stem. 



In other cases, stipules develop which are generally regarded as 

 divergences from the base of the leaf. Sinnott and Bailey (3 8) have 



