22 MINNESOTA BOTANICAL STUDIES. 



ing endosperm tissue, and is surrounded by a layer of disorganized 

 cells 120 niic.-500 mic. in thickness. A portion of this broken-down 

 endosperm becomes enclosed by the cotyledons and is to be seen even 

 in the mature seed as a small mass of structureless material between 

 their bases. 



DiFFERENTATION OF TiSSUE IN THE EmBRYO. 



During the early stages of its development, up to the period 

 when the cotyledons appear, the only differentiation in the tissues 

 of the embryo is that previously noted by which first the basal cell 

 or cells and progressively those above this lose the power of further 

 division, and become somewhat vesicular, forming the suspensor. 

 The body of the embryo throughout this stage of development con- 

 sists of an entirely undifferentiated mass of cells with dense gran- 

 ular protoplasm and large nuclei. Cell division occurs abundantly 

 throughout this mass of tissue, and apparently in no regular order 

 or direction. Spindles may frequently be found close together and 

 running at various angles to one another. There is not even a 

 differentiated epidermis and while most of the cell division in the 

 superficial cells is radial, tangential divisions have been observed in 

 these cells in embryos over 300 mic. in length, and are plainly indi- 

 cated in yet older ones. 



About the time that the cotyledons begin to develop, the first 

 signs of tissue differentiation appear. This seems to be brought about 

 by a specialization in the direction of cell division in the various 

 parts of the embryo. In the axial region the cells divide both trans- 

 versely and longitudinally, the latter direction predominating, while 

 the cell division is accompanied with comparatively little cell growth, 

 the peripheral cells divide mainly transversely, and undergo consid- 

 erable cell growth. This results in the formation of a plerome 

 strand of comparatively small, cylindrical and irregularly arranged 

 cells surrounded by a periblem region of larger and less cylindrical 

 cells which are arranged in somewhat irregular longitudinal rows. 

 These rows curve inward at the suspensor end of the embryo, and 

 appear to radiate from a region in the axis of the embryo just 

 above the suspensor, which becomes the growing point of the root 

 (Fig. 15, PI. V). Apparently this growing point functions some- 

 what during the further development of the embryo, but growth is 

 not localized there, and cell divisions are scarcely more numerous 

 there than elsewhere. 



