220 



BOTANICAL GAZETTE 



[SEPTEMBER 



are they laid down at regular intervals of time. The plant which 

 was studied was more than a century old and yet had but 3 vas- 

 cular cylinders. Doubtless the cortical cylinders are related to 



certain activities of the plant 

 alternating with long periods 

 of rest which may vary 

 greatly in point of duration. 



Summary and conclusions 



1. The paper deals with 

 the adult stem of Cycas media, 

 particular attention being 

 paid to the xylem and phloem 

 details of the normal and first 

 cortical cylinders. 



2. Not all the vascular 

 cylinders are of equal longi- 

 tudinal extent. Only the 

 normal one begins its differ- 

 entiation as high up as the 

 meristem, the others begin- 

 ning their differentiation suc- 

 cessively lower, and each one 

 in the cortex outside the next 

 inner cylinder. The normal 

 cylinder, therefore, is the only 

 one which would be expected 

 to originate with a procam- 



bium, and the only one which could develop protoxylem and 

 protophloem. 



3. Following up these expectations, both protoxylem and pro- 

 tophloem were found to have been developed during the early 

 activities of the normal cylinder. Protoxylem elements are usually 

 scalariform, although hints of spiral tracheids are more or less 

 frequent. Primary xylem is scalariform and secondary xylem is 

 characteristically pitted. 



Fig. 11. — Cycas media: transverse section 

 of stem, showing tip of phloem belonging to 

 second cylinder; c, cortical cells; b, suberized 

 bast fibers; X235. 



