700 SECONDARY GROWTH IN THICKNESS 



the circle of primary bundles, of a cambium which produces secondary 

 xylem and phloem in strictly normal fashion. The medullary rays, 

 which are unusually broad and altogether massive, are traversed by the 

 leaf-traces and by the radial anastomoses of the primary network. 

 Since this network permanently retains its individuality, both radial 

 and tangential anastomoses must constantly elongate as secondary 

 thickening progresses ; one result of the continued extension is, that 

 the tracheides in these anastomoses ultimately undergo deformation 

 and disruption. The genera Zamia, Dioon, and Stangeria never 

 develop more than a single cambium. In Cycas and Encephalartos, on 

 the other hand, a succession of more or less concentric cambial layers 

 arise near the periphery of the secondary phloem (according to 

 Constantin and Morot in the pericycle). Each of these successive 

 " adventitious " cambial zones remain active for several seasons. 



Regarding the products of cambial activity in the Cycads, it may 

 be stated, that the secondary xylem consist of tracheides with scalari- 

 form-reticulate thickenings, or with transversely elongated bordered pits, 

 together with a certain amount of xylem-parenchyma ; the secondary 

 phloem comprises bast-fibres as well as sieve-tubes. 



In the genus Cycas itself, a secondary network of bundles con- 

 tinuous with the primary network appears in the cortical paren- 

 chyma. In Encephalartos, on the other hand, a dense weft of secondary 

 vascular strands becomes differentiated in the pith ; the peripheral 

 strands of this medullary system become connected with the inner 

 surface of the woody cylinder. 



By way of a supplement to the preceding discussion of secondary 

 growth in thickness in Phanerogams, a brief account may be given of the 

 analogous processes which take place in certain Cryptogamic forms. 3N 



In two genera of the Ophioglossaceae, namely, Botrychium and 

 J '[> hninthosfachys, the primary central cylinder becomes greatly dilated 

 by the differentiation of a parenchymatous pith : in addition, a 

 meristeinatic layer sooner or later appears between hadrome and 

 leptome ; the amount of secondary tissue produced by this cambium 

 is, however, limited, and consists entirely of tracheides cut off on the 

 inner side. The " cambium " which occurs in the tuberous stems of 

 Isoetcs apparently only serves to increase the amount of storage-tissue ; 

 here the majority of the secondary elements are extracambial, only a 

 small number being incorporated in the axile strand. Secondary 

 tracheides have been observed in Isoetcs lacusfris and in /. Durieui, 

 but even here seem to be few and far between. 



Among Algae secondary thickening is confined to the stipes of the 

 Laminakiaceae. In some members of this family it is the superficial 

 photosynthetic layers that cut off new tissues towards the inside 



