Asn.EPIADACEAE. 



327 



Periploca. Silk Vine. 

 (Family Asclepiadaceae). 



Soft-wooded twiners with milky 

 sap: deciduous. Stems terete, 

 moderate: pith round, excavated. 

 Buds rather small, solitary, near- 

 ly concealed by the leaf-bases. 

 Leaf-scars opposite, raised, shriv- 

 eled or elliptical with a single 

 crescent-shaped bundle-trace: sti- 

 pule-scars lacking. 



Winter-characters of P. graeca 

 are figured by Schneider, f. 109. 



Several herbaceous milkweeds 

 are twining plants and become 

 troublesome weeds when they get 

 a foothold in orchards or beside 

 fence-posts where it is hard to 

 dislodge their strong perennial 

 roots. The silk vine, which is the 

 only woody representative of the 

 family that is hardy well into the 

 North, is a strong climber. 

 A technical distinction between the closely related fami- 

 lies Apocynaceae and Asclepiadaceae is found in the powdery 

 or granular pollen of the former and the coherent pollinia of 

 the latter, familiar to every student of milkweed pollination 

 and to every close observer of bees and other insects, to which 

 the pollen masses become attached. In Periploca these pol- 

 linia are less firm than in most genera of the family. A 

 typographic slip in one case has caused the apocynaceous pol- 

 len to be called glandular, tempting one to parallel Engel- 

 mann's impatient exclamation when what he wrote for glu- 

 tinous pollen appeared in type as gelatinous, "but who ever 

 heard of gelatinous pollen?" 

 Stems glabrescent from somewhat puberulent. P. graeca. 



