CELL COMMUNITIES: A CHAPTER ON TISSUES 



89 



and has transfusion tissue present in connection with its rather widely 

 separated bundles, though more feebly developed than in most Conifers. . . . 

 It is well known that the bundles of the leaf of Cycads have a structure 

 peculiar to this order and not found in any other living group of plants. 

 Towards the lower surface of the lamina is placed the phloem ; next comes 

 the ordinarjr xylem, which is formed by the cambium in a centrifugal 

 manner ; on the inner side of the secondary wood there may or may not 

 be a few elements of primary centrifugal wood, and then follows the 

 protoxylem consisting of narrow, elongated, spirally or reticulately thickened 

 elements. Farther, beyond the protoxylem, i.e. between this tissue and 

 the upper surface of the leaf, occurs another strand of xylem, primary in 

 origin, and of much greater development than that of the centrifugal 

 wood; it is centripetal in 

 development, i.e. its ele- 

 ments are formed succes- 

 sively from the protoxylem 

 towards the upper surface 

 of the leaf ; it is charac- 

 teristic of the Cycadese. 

 Typical transfusion tissue 

 occurs at the side of the 

 bundle, and this is seen to 

 be in intimate connection 

 with the centripetal xylem. 

 In the petiole the structure 

 of the bundles is the same, 

 though their orientation is 

 different. In other Gymno- 

 sperms and all Angiosperms 

 this tissue is, so far as 

 hitherto observed, absent 

 from the vascular bundle." * 

 Here we conclude. We 

 have travelled together over 

 a good deal of ground, and 

 the physiologicalfacts which 

 have come before us must 



* Mr. W. C. Worsdell, from 

 whom these remarks are quoted, 

 read a paper dealing with this 

 subject at length before the Lin- 

 nean Society in November, 1897 ; 

 the paper will be found in vol. v. 

 of the botanical Transactions of 

 that Society. 



FIG. 121. CONES OF A CYCAD. 



[E. step. 



The Cycads were abundant in Jurassic and Wealden times, but are now 

 very few. They are believed to have been the starting-point from 

 which all our plants with conspicuous flowers (Angiosperms) originated. 



