10 



MORPHOLOGY OF THE ANGIOSPERMS 



is often cited as an example of this reduction. In the Oleaceae, Syringa 

 has both simple- and compound-leaved species; Forsijthia may have 3- 

 foliolate leaves among the simple ones. Fraxinus, which usually has 

 many leaflets, has two species with only one, and unifoliolate forms oc- 

 cur in other species. The Proteaceae perhaps show the most complete 

 story of lamina reduction. Tropical genera with the most primitive floral 

 characters — for example, Placosperma, Hicksheacliia, Austromuellera — 



have large compound leaves; the 

 genera of more temperate climates 

 show many types of reduction, as- 

 sociated with xerophily, to small 

 and secondarily simple. 



The leaves of Baiihinia and re- 

 lated genera show both loss of 

 leaflets and fusion of a surviving 

 distal pair to form a solitary pseu- 

 doterminal leaflet. All stages of the 

 union are seen within the genus 

 Baiiliinia — from two free leaflets 

 to one apparently simple blade 

 (Fig. IC to H). Involved in this 

 union is the free tip of the rachis 

 between the two leaflets. (Similar 

 fusion occurs in inflorescences 

 where two lateral flowers unite to 

 form a pseudoterminal flower — 

 Lonicera spp.) In most stages of 

 leaflet union, the fusion is con- 

 genital, and in the species with 



compound Teaf of Carija buckleyi \iu. completely fused leaflets, there 



arhansana, showing acropetal sequence in 



leaflet formation from lateral primordia. 



c, cortex of leaf axis; h, hair; lU to IL, 



primordia of lateral leaflets; tl, terminal 



leaflet. (After Foster.) 



Fig. 2. Longitudinal section of developing 



remains little or no evidence in 

 form or venation of the double 

 nature of the "terminal" blade. 



The compound leaf, as an ad- 

 vanced type, is believed to have 

 arisen by the evolutionary dissection of the simple leaf. In ontogeny, the 

 leaflets develop as do the lobes of a simple leaf — by the development of 

 lateral primordia on the median axis (Fig. 2). The development of the 

 compound leaf of the palms is wholly different — by an ontogenetic split- 

 ting of the primordium ( Fig. 18 ) . 



In gross venation, pinnate venation appears to be primitive, the 

 palmate derived. Many transitional forms occur. Evidence that pinnate 

 venation is primitive is found in the anatomy, especially in the 



