30 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



form the initial cells for the periblem of the root and are 

 continuous with the periblem of the middle segment. Fi- 

 nally, the segment below this is simply a continuation of 

 the epidermis of the rest of the embryo, which thus extends 

 without interruption completely around it. About as soon 

 as the tissue-systems are clearly defined, there begins the 

 external differentiation of the embryo into the primary or- 

 gans of the young plant. 



VIII. The Cotyledon. 



The cotyledon of Naias, as in other typical Monocotyle- 

 dons, is strictly terminal and is the product of the whole of 

 the terminal segment of the embryo. There is early a 

 marked enlargement in the middle region of the embryo 

 (figs. 69, 70), corresponding to the stem-segment, while the 

 cotyledonary portion, although elongating, does not increase 

 so rapidly in diameter. This enlargement of the stem-seg- 

 ment is largely on one side and results in the formation of 

 a prominence at this point (figs. 70, 71, st) — the apex of 

 the stem of the young plant. It is evident that this is here 

 strictly lateral in origin. By the growth of the stem-apex, 

 which is quite active from the first, the cotyledon is pushed 

 somewhat to one side. The growth of the cotyledon, which 

 is at first terminal and due largely to the growth of the 

 tissues at the apex, is soon replaced by the formation of a 

 zone of meristem at the base, which not only causes the 

 subsequent growth in length, but by the active multiplica- 

 tion of the cells laterally forms a sheath-like stipular growth, 

 surrounding the base of the cotyledon (fig. 69) and finally 

 completely concealing the stem-apex, which comes to lie in 

 a cavity completely enclosed by the overlapping edges of the 

 leaf-base (figs. 72, 73). In median sections of the young 

 cotyledon (fig. 70, h'), the separation of the tissue-systems 

 is evident, but the plerome is never so well developed as in 

 the stem and root, and this corresponds with the slight de- 

 velopment of the vascular bundle in the mature cotyledon. 

 The fully grown cotyledon is about half the length of the 

 whole embryo (fig. 73)- 



