lOO 



MORPHOLOGY OF GYMNOSPERMS 



D •-■/ 



primary strand. The two lateral strands that enter a cotyledon are 

 branches of the two primary strands that are connected with the two 

 intercotyledonary angles or poles, and therefore are secondary strands. 

 The primary strand from an intercotyledonary angle follows a very 

 short radial course and then forks, the branches taking a tangential 

 course in opposite directions and passing into opposite cotyledons. 

 Each primary intercotyledonary strand, therefore, sends a branch 



^ into each cotyledon; while each 

 primary cotyledonary strand is 

 connected with a single cotyle- 

 don as its median strand. This 

 median strand usually does not 

 fork until well within the coty- 

 ledon of Ceratozamia; in some 

 other cycads it forks sooner; 

 and in others it is said to be 

 double from the beginning. 

 This variation in the forking 

 . . ,. of the median strand accounts 



Fig. 76. — Ceratozamia mexicana: diagram 



representing connection of cotyledonary bundles '-^^ '•'^^ TCportS OI tnrce Or lOUr 



with the vascular plate; A, B, C, D, the four vascular bundles occurring at 



main cotyledonary bundles; C, median bundle ^j^^ ^^^^ ^f ^^^^ COtyledon. 



of developed cotyledon; D, median bundle of • 1 i 1 



aborted cotyledon; a, a^, lateral bundles of ^^ Ceratozamia the lateral 



aborted cotyledon; b, b', lateral bundles of Strands and also the bifurcated 



developed cotyledon; /', /^ js, /4, the four prin- median Strand branch, so that 



cipal groups of foliar bundles; c, cotvledon; , ^ ^ ^ i^- 



; r r ^x c u A / X the cotyledon becomes multi- 



/, leaf.- — After Sister Helen Angela (59). -^ 



fascicular. The xylem of the 

 cotyledonary strands is mesarch at the base of the cotyledon, but 

 becomes exarch in the upper part. 



In the early stages of the seedling several groups of extrafascicular 

 cambium appear (fig. 78), but in seedlings two years old only the 

 slightest trace of them can be found. The stem develops as a sympo- 

 dium, and the number of vascular strands entering successive leaves 

 increases, sometimes with great regularity, as from three in the cotyle- 

 don to seven in the fourth leaf. The origin of these leaf strands is 

 exceedingly complex, as they represent a few of the ultimate branches 

 from four (or three by the fusion of two) original foliar bundles con- 



