Butters: seeds and seedling of caulophyllum. 23 



The plerome and periblem regions thus differentiated are not 

 sharply distinguished from each other. Although each region is 

 characterized by certain peculiarities of cell behavior, these peculiar- 

 ities are not absolutely fixed, and there is never a sharp line of de- 

 marcation between the two regions. 



Even in ripe seeds there is scarcely any differentiation between 

 the cells of the plerome, the characteristic differences between xylem 

 and phloem appearing after growth is resumed. 



As the cotyledons develop, plerome strands become differentiated 

 in them in a manner similar to the formation of plerome in the axis. 

 There are three of these strands in the lower part of each cotyledon. 

 They branch, and become more numerous in the broad upper por- 

 tion of the cotyledon, while at the base they fuse into the general 

 plerome cylinder of the axis. 



The epidermis becomes very slowly differentiated, and is for the 

 most part but slightly specialized even in the ripe seeds. Its cells 

 are somewhat smaller than the underlying periblem cells, and their 

 protoplasm is more dense. The epidermis becomes most highly 

 specialized in the upper parts of the cotyledons. On the inner sur- 

 face of their concave distal parts several stomata are developed 

 (Fig. 17, PI. V). These have the usual form of such structures. 

 Their presence is especially interesting in view of the fact that the 

 cotyledons never become epigean. 



The cells immediately below the growing point of the root, and 

 next to the suspensor divide several times transversely, forming 

 the primordium of the root cap (Fig. 12, PI. V et seq.). The de- 

 velopment of this structure extends laterally about the convex end 

 of the periblem, the cell divisions being always parallel to the sur- 

 face of the latter tissue, and finally it extends across the whole width 

 of the embryo, involving tangential divisions of the epidermal cells 

 where it comes out to the surface of the embryo. During its fur- 

 ther development the root cap thickens considerably, especially in the 

 axis of the embryo where it is ten to twelve cells thick in the ripe 

 seed. 



The cells at the distal end of the axis of the embryo, which event- 

 ually give rise to the growing point of the stem, change very little 

 during the maturing of the seed. In the ripe seeds they appear as 

 a low mound of irregularly arranged isodiametrical cells covered 

 by a fairly distinct epidermis (e, Figs. 15 and 16, PI. V). 



