66 WRIGHT : THE GENUS DIOSPYROS 



advanced in the median cotyledonary trace as compared with 

 the laterals. 



In the short axis of the embryo it is difficult to fix on the 

 exact area at which lignification first appears. Examination 

 of later stages shows the maximum lignification of the coty- 

 ledonary xylem at the collet area, and that the lignification 

 subsequently spreads downwards into the v primary root and 

 upwards into the hypocotyl. The collet area is also the first 

 to show a vascular cambium and the maximum parenchy- 

 matous system ; it therefore appears to be the centre of 

 activity of the seedling axis. 



The fact that the lignification of the cotyledonary xylem 

 reaches the maximum at the collet area mar, however, still 

 allow of the original starting point being at the cotyledonary 

 node, and the enhanced lignification at the collet may be a 

 secondary development associated with the requirements of 

 the seedling subsequent to the purely embryonic stage. In 

 the embryo the conducting systems are functioning in the 

 conveyance of nutritive materials from the endosperm to the 

 primary axis, and there does not therefore appear to be any 

 necessity for lignification to commence at the collet area. 

 There is the possibility that the direction of lignification 

 may be purely responsive to the requirements of the 

 seedlings, in which case no evidence can be obtained of 

 the direction of differentiation of the original xylem 

 elements. 



The lignification is completed throughout the hypocotyl 

 and primary root of the embryo within such a period, that 

 prior to the apex of the primary root presenting itself at the 

 micropyle a continuous series of lignified elements exist 

 from root to cotyledons. 



The epicotyledonary strands when present in the primary 

 root show a definite direction of differentiation. 



The phloem of each epicotyledonary strand begins at the 

 epicotyl leaf and spreads downwards into the hypocotyl and 

 root. This direction of differentiation is obviously the best, 



