io6 



MORPHOLOGY OF GYMNOSPERMS 



their elements become continuous with those of the four poles of the 

 root, which is at first tetrarch, but may become triarch toward the tip. 

 Each cotyledon contains eight or ten vascular strands, with alternating 

 mucilage ducts, the strands being all derived from the branching of 

 three main bundles that join the central cylinder. In the young 

 stem four large groups of leaf traces alternate with the four cotyledo- 

 nary groups, and just above the cotyledonary node they close together 



„ to form the central 



cylinder. In this case, 

 -ft also, the course of the 



girdle is evident in its 

 procambial stage; and 

 all the strands change 

 from the endarch con- 

 dition to the mesarch 

 and finally the exarch in 

 passing from the cen- 

 tral cylinder toward the 

 tip of the cotyledon or 

 leaf, and transfusion 

 tissue is abundant in 

 close connection with 

 the centripetal xylem. 

 In Microcycas also there 

 are remnants of a broken 

 up cambial zone in the 

 cortex of the young stem, not so distinct as in Ceratozamia and never 

 forming an extrafascicular bundle. 



The seedlings of these four genera may be regarded as representa- 

 tive of the seedlings of cycads in general, and the situation may be 

 summarized as follows: At the cotyledonary node there appears 

 a quadrangular protostelic plate or a siphonostelic grouping of four 

 sets of cotyledonary strands (Microcycas), in both cases connecting 

 with the four poles of the tetrarch root, which later may become triarch 

 or even diarch. At the base of the cotyledon three or four vascular 

 strands occur, connected with three of the poles of the quadrangular 

 vascular plate. In Zamia and Dioon edule these strands seldom 



Fig. 82. — M'srocycas calocoma: semi-diagram- 

 matic transverse section of the central vascular sys- 

 tem of the seedling above the cotyledonary node, 

 composed of four groups of leaf traces (/', j', /s, /4); 

 A, B, C, D, main cotyledonary traces. — After 

 Sister Helen Angela (65). 



