14 



MORPHOLOGY OF THE ANGIOSPERMS 



the leaves of seedlings show stages in the development of the sheathing 

 base by the merging of adnate stipules with the base of the petiole 

 (Fig. 4). 



In many families, there is evidence of reduction of the stipules, asso- 

 ciated with the loss of their function as protective structures for meri- 

 stems and young leaves; in the Leguminosae, Droseraceae, Onagraceae, 

 Cornaceae, and many others, there are taxa with poorly developed or 

 vestigial stipules and other taxa without stipules. In some species, 

 stipules are present in the upper leaves, absent in the lower; in other 



E F 



Fig. 4. Sketches of stipules of Potamogeton showing stages in connation of a pair of 

 stipules to form a stipular sheath. A, P. perfoliatus; B, C, P. lucens; D, E, P. natans; 

 F, P. crispus. (After Monoijer.) 



species, present in the lower, absent in the upper leaves. In Tropaeolum, 

 they are present only in seedlings. Stipules appear to be a disappearing 

 feature of the angiosperm leaf. In dicotyledons, they have apparently 

 been lost in most of the higher families; in the monocotyledons, they 

 may survive as a part of the sheathing leaf, an adaptation to the domi- 

 nant monocotyledon habit. 



Anatomical evidence for the primitiveness of the stipulate condition 

 seems contradictory. Stipulate taxa commonly have trilacunar nodes; 

 they rarely have unilacunar nodes with an odd number of traces. If the 

 unilacunar node with two traces is primitive (Fig. 6A), as now seems 



