Lyon: the embryogeny of ginkgo. 285 



figs. 52-5 j). Usually, the four secondary protoxylem-bundles, 

 adjacent to those of the cotyledon-traces, contribute to the latter 

 small groups of elements which unite with them just before they 

 pass out to the cotyledons (Diagram C. -And Jigs. 55-61). 



While the plan of transition outlined above is often followed 

 to a nicety in a seedling, exceptions are very numerous. The 

 length of the transition-region varies considerably in different 

 seedlings. As a rule, the cotyledon-traces do not leave the stele 

 at the same height ( figs. 51, 61), and the reduction of the traces 

 into the two root-bundles may take place at very different levels. 

 It is often impossible to recognize distinct bundles in the traces 

 through the absence of protoxylem, or the indiscriminate ming- 

 ling of protoxylem and metaxylem elements. The sharp limi- 

 tations of protoxylem and metaxylem indicated in the diagrams 

 are rarely met with in a series of sections. 



The origin of the stem-bundles in the transition-region, is 

 usually after the manner already described {flg's- 50-57, 60, 

 61). Occasionally, seedlings are met with, however, in which 

 the traces of the first two leaves are prominent in the transition- 

 region, and, with the four bundles of the cotyledon-traces, form 

 a hexarch stele, which may become resolved, lower down, into 

 such a tetrarch stele as seen in fig. 63. Such leaf-traces are 

 often distinctly mesarch. Then again, the trace of a single 

 leaf may continue prominent through the transition-region, and 

 produce a triarch stele for a considerable distance in the root. 

 In a tricotyledonous seedling, the primary root is persistently 

 triarch. Secondary roots are always diarch {Jig. 62). 



Secondary thickening in the stem and root of Ginkgo takes 

 place in the ordinary manner. The position of the cambium in 

 the root is shown in the diagrams and in figs. 5 0—5 7 , 60-64. 

 Below the transition-region, the pith of the root may be partially 

 or almost wholly replaced by metaxylem {Jigs. 62-64). The 

 centrifugal xylem becomes continuous in places with the metax- 

 ylem, but it rarely if ever comes in contact with the protoxylem. 

 It is often separated from the latter by only a single layer of 

 pith-cells {Jig. 62). The growing-point of the root ( fig: 65) 

 presents essentially the same structure as those of the cycads 

 and conifers which have been described by De Bary ('84). The 

 cork-cambium of the stem arises directly beneath the dermato- 

 gen ; in the root its origin is deeper seated. 



The structure of the cotyledons of the seedling is illustrated 



