84 



THE STRUCTURE OF ECONOMIC PLANTS 



Development of Stipules. — Stipules may be considered mor- 

 phologically as integral portions of the leaf, but relatively little 

 work has been done in regard to the details of their development. 

 Early studies on stipules w^ere concerned primarily with speculation 

 as to the relation of the stipule to the leaf; and, more particularly, 

 the consideration of any evidence based on the presence or absence 

 of stipules which might serve as an aid in phylogenetic inter- 

 pretations. Considerations of this nature have been reported by 



•--si 



— L-1 



A B C 



Fig. 31. Development of the pinnate leaves of the Umbel liferae. A, B, of Pastinaca sativa; 

 C, of Levisticum officinale. In A, the stem tip and youngest leaf L-i are shown. The leaf, 

 L-i, has commenced pinnation. In B, the primary pinnation of a single leaf is shown. In 

 C, the leaflets of the first order, /', have given rise to secondary leaflets, /"; st, stem tip. 

 (Redrawn and adapted from Sachs, Textbook of Botany, Clarendon Press.) 



Domin (ix), Sinnott and Bailey (38), Schrodinger (34), and more 

 recently by Ponzo (xS). 



Ontogenetic studies have been limited, and have received but 

 brief mention in connection with general foliar studies. Cross (10) 

 has described the development of the stipules in Morus alba and 

 states that they "undergo a decidedly different ontogenetic devel- 

 opment from that of the foliage leaf, but they exhibit certain fea- 

 tures of development which are strikingly suggestive of bud scale 

 histogenesis." Following expansion, stipules sometimes absciss 

 early, before the foliage leaf attains its full size. Abscission 

 is not a constant character of these structures; for, in many cases 

 as in Pisum, they are not only persistent, but perform an important 

 role in photosynthesis. 



