Lyon: THE EMBRYOGENY OF GINKGO. 279 



4. Embryonic Tissue-Systems. As the blastema grows out 

 from the protocorm, its superficial cells are irregular, and divide 

 by periclinal as well as anticlinal walls (Jig- 4). This condi- 

 tion prevails even after the stem apex has become quite distinct 

 (jig. 5). Later, with the broadening of the apex, which causes 

 a rapid increase of surface, periclinal divisions become less fre- 

 quent. They, however, continue to occur for some time in the 

 most central region of the stem apex, and can often be detected 

 here in nearly mature embryos. Superficial cells of the coty- 

 ledon and leaf-meristems have been found dividing periclinally. 

 In regions, other than meristems, the dermatogen appears to be 

 morphologically distinct from the subjacent tissue. 



The plerome and periblem can only be recognized as regional 

 in the embryo, for they are not sharply marked off from each 

 other. The first procambial tissue is differentiated in connec- 

 tion with the cotyledons, passing from these straight back into 

 the body of the embryo (Jig. j)- The next procambium-strands 

 arise, in like manner, in connection wtth the first plumular 

 leaves ; but since these are closer together than the cotyledons, 

 their procambium-strands are closer together. The shape of 

 an embryo's stele is determined by these two sets of procambial 

 strands. Hence, if there are two cotyledons, the stele will be 

 ellipsoidal (figs. 13-15) ; but if there are three cotyledons, it 

 will be triangularly prismoidal (fig. 18). Attheroot-meristem, 

 an ellipsoidal plerome narrows down into a broad wedge (Jigs. 

 n, 12) ; while in the same region a prismoidal plerome narrows 

 down into a blunt pyramid. A single procambium-strand enters 

 each cotyledon. As it passes upward in the cotyledon, this 

 strand, usually but not always, divides once. A few pro- 

 toxylem-elements are often differentiated in the cotyledon- 

 bundles before intraseminal growth stops. 



In both plerome and periblem numerous secretory vessels are 

 formed through the breaking down of cells in longitudinal 

 rows (Jigs. 10-12). Many spherical resin-reservoirs also arise 

 in the cortex of the stem, cotyledons and leaves by the disor- 

 ganization of masses of cells (Jig's. 10-20). In mature embryos 

 the cells of the cortex are packed with starch. 



5. Polyembryony. It often happens that when the oosphere 

 of one archegonium of a gametophyte is fertilized, the oosphere 

 of the other is also fertilized and two protocorms develop. Occa- 

 sionally gametophytes will be found with three archegonia ; 



