208 



MR. A. S. HOEKE — A CONTRIBUTION TO 



ovule, being erect, is only slightly anatropous and hence can be regarded as exhibiting 

 the very earliest stage in the evolution of the anatropous ovule. 



The stage in evolution evident in Juglans is followed by the complete inversion of the 

 nucellus, owing to the curvature of the raphe as in the Cactacese and Saxifragaceae, and 

 further by the congenital union of the raphe and integument as exemplified by the 

 HosaceaD. This phase of evolutionary advance is characterized by the separation of 

 several anatropal types. Two types obtain in the E-osacese: one '* ascending," with the 

 nucellus directed upwards as in Cerasus Juliana ; the other, " descending," with the 



Texfc-fis:. 13. 



unin^- 



£ 



Diagrams illustrating the structure of the ovule. 

 Helwingia ruscifol'm ; C, Davidia involucrata ; I 



a(?=aflplacental side; a6=abpiacental side; c = chalaza; c.sac = embryo-sac; {n<=integument ; {ni.r= cell-rows 



of integument; in.int=inj\ev integument; out.int = outer integument; Mp.inf = upper margin of integument: 

 un.mt = lower margin of integument ; ?=limit of integument ; m =micropyle ; w=nucellus ; ^=pivot cell. 



nucellus directed downwards, as in Cr 



Oxyacantha and JPyrus Mains (Text 



fig. 13, A). The former type is not far removed from the '' dorsal " ovule which 

 prevails throughout whole families, such as the Araliacese, whilst the ventral ovule in 



Cornacese (Text-fig. 13, E, Aucub 



ft 



B, Set win a 



ably an advance upon the 



descending " type. Specialized forms of the '' dorsal " and *' ventral " ovule 



found 



m 



Davidia and Helwingia respectively (Text-fig. 13, C and B), where the integument 



{int) exists as a separate organ for only a short distance below the summit of the 



to the line marked I in the figures 



