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



in the central part of the embryo (compare figs. 66 and 68). 

 The tissues, therefore, are strictly continuous in the two. 

 The first transverse walls, however, in the primary plerome 

 and periblem cells of the middle segment, effect the separa- 

 tion of these tissues in the stem and root. At this period, 

 and for a long time after, the two terminal cells of the plerome 

 (seen in longitudinal section) abut against the central cells 

 of the segment below, and the latter form at once the peri- 

 blem initials of the root. 



The differentiation of the tissues becomes very plain as 

 the root grows, and the arrangement of the tissues at the 

 apex is found to be quite constant. In a median section 

 (figs. 75, 76) the tissues are arranged as follows: Bound- 

 ing the section is a single layer of epidermal cells, which, at 

 the apex, are continuous with the cells formed by the division 

 of the lowest segment of the embryo. Between the latter 

 and the primary suspensor are two or three short cells, 

 formed by transverse division of the lowermost of the pri- 

 mary embryonic segments. These are usually considered 

 as belonging to the suspensor, and may perhaps be called 

 suspensor cells ; but it must be borne in mind that they are 

 not derived from the primary suspensor, but from the 

 embryonic half of the young embryo. 



Below the epidermis and separating it from the end of the 

 plerome cylinder is the single layer of periblem initials. The 

 plerome cylinder usually shows in a longitudinal section a 

 single terminal cell, which is probably to be considered as 

 the initial for the plerome. The latter shows in longitudinal 

 section about three rows of cells, although this is not so 

 evident in the younger embryo, where the apex often shows 

 but two rows of cells, corresponding to the primary arrange- 

 ment. Immediately back of the apex each periblem cell 

 divides by a longitudinal wall, thus forming two layers of 

 periblem cells, and the inner layer is soon divided again in 

 the same way; but the hypodermal layer remains for the 

 most part undivided and may be recognized as such in the 

 older root, all the subsequent thickenings of the root being 

 due to the divisions of the inner layers of the ground-tissue, 



