Yampolsky: Stiuly of OU palni. 127 



The formation of wood goes on herc coiiicident witli the exposure of the 

 leaf to the outside eiiviroiiment. The mature leaf is a highiy résistant orgaii 

 which can be subjected to the severest storms witiiout separating it from 

 the rest of tiie plant. 



Discussion. 



AUhoiii^h ail the foregoing investigators hâve very amj^ly discussed 

 the literatiire of their time, it does not seem to me to be out of place, in 

 the light of the évidence that has been advanced, to discuss the results of 

 the older investigators from the point of view that has been submitted hère. 



The first critical work on the palm leaf was that of VON MOHL (^^). 

 It is truly remarkable, taking into considération the comparatively primitive 

 methods, that such good observations were made on leaf development. 

 We hâve already summarized his observations in the historical part of this 

 pâper. Unfortunately VON MOHL's observations uid not go back far enough 

 from the stage that shows the appearance of furrows and slits. Between 

 the appearance of the young undifferentiated leaf and the stage in which 

 the furrows .appear the critical stages are to be found. That too we hâve 

 fully discussed. it has been pointed out that the furrows that VON MoHL 

 described are really the developing leaflets within the leaf blade proper and 

 that the furrows indicate the areas between adjacent developing leaflets it 

 should be emphasized, however, that VON MOHL states that in this stage 

 the leaf is yet undivided. it is, according to him, due to the development 

 of slits that the division of the lamina occurs. The leaflets are ultimately 

 set free by the drying up and falling off of the loose parenchyma which 

 holds them together. 



The undivided part of the leaf blade differs from a pubescence in its 

 mode of origin inasmuch as it is not an outgrowth of the upper surface 

 but an actual part of the leaf tissue. This keen observation of VON MOHL's, 

 GOEBEL and ElCHLER misinterpreted by stating that the undivided part of 

 the leaf is a secondary structure. 



Trecul {^^) speaks of a pellicle enveloping the very young leaf. When 

 this is removed the parallel ridges on both surfaces of the leaf are seen. 

 Trecul did not observe this pellicle in earlier stages. This pellicle according 

 to him is a secondary development arising from a gelatinous translucent 

 substance around the leaves. Trecul's work did not contribute to the 

 knowledge of the development of the palm leaf because he too did not 

 hâve a complète séries of stages under observation. 



It was HOFMEISTER ('-^) who made the broad generalization that 

 folding occurs owing to the limited space in which the young growing 

 leaf develops. The séparation of the leaflets results from the death of a 

 definite tissue between the segments causing complète or partial libération 

 of the segments. This generalization has up to the présent been accepted 



