Structure. 



57 



slightly wider than deep to slightly deeper than wide. In N. iuberosa the 

 latter shape occurs near each end of the petiole, while near the middle the 

 cells are approximately square in section. The bases of the hairs, Fig. 

 25, (a), (b) are now seen to consist of a short epidermal cell whose inner 

 end is about flush with its neighbors, surmounted by one or two shallow, 

 disc-like oells which come up level with the outer surface. Two disc-like 

 cells are constant in N. rubra and zanzibariensis X and occasional in N. 

 tuberosa and amazonum, only one being found elsewhere. The disc-like 

 cells are very shallow, one-third to one-fourth the depth of the epidermis 

 in N. odorata and its varieties, but in other species they occupy about half 



ro 



OOOfiOo&qb 



1 



0, 



(a,) 



Fio. 25.--Collenchyma in petioles, 

 (a) N. sturtevantii ; (b) N. flava ; b, basal cells of a hair. 



the depth of the epidermis. A thick cuticle is always present, and was 

 seen in N. tetragona to be distinctly lamellate. 



Beneath the epidermis there is a single subepidermal layer, distin- 

 guishable in all but N. tetragona by its thin walls, and in N. rubra by 

 the larger size and somewhat columnar arrangement of the cells. Under 

 this layer is a zone of collenchyma (Fig. 25), whose cells remain of small 

 or medium size but are followed internally by much larger fundamental 

 cells in N. mexicana, flava, rubra, amazonum, and the middle parts 

 of the petiole of N, zanzibariensis X , but which become gradually larger 

 inward and pass more or less insensibly into the fundamental tissue in 

 N. odorata, tuberosa, tetragona and the lower part of petiole of N. zanzi- 

 bariensis X . This zone is about 3 cells wide in N. flava (a small petiole), 

 4 in N. tetragona, mexicana, amazonum, 5 or 6 in the upper and lower 

 ends of N. tuberosa petiole and the middle of N. zanzibariensis X , 6 or 7 

 in the middle of N. tuberosa, and 8 in N. odorata, rubra, and the two ends 



