VIl] 



OF HEXAGONAL SYMMETRY 



335 



A very beautiful hexagonal symmetry, as seen in section, or 

 dodecahedral, as viewed in the solid, is presented by the cells 

 which form the pith of certain rushes (e.g. Juncus ejfusus), and 

 somewhat less diagrammatically by those' which make the pith 

 of the banana. These cells are stellate in form, and the tissue 

 presents in section the appearance of a network of six-rayed 

 stars (Fig. 133, c), linked together by the tips of the rays, and 

 separated by symmetrical, air-filled, intercellular spaces. In thick 

 sections, the solid twelve-rayed stars may be very beautifully seen 

 under the binocular microscope. 



: c 



Fig. 133. Diagram of development of "stellate cells," in pith of Juncuf>. 

 (The dark, or shaded, areas represent the cells; the Ught areas being the 

 gradually enlarging "intercellular spaces.") 



What has happened here is not difficult to understand. 

 Imagine, as before, a system of equal spheres all in contact, each 

 one therefore touching six others in an equatorial plane ; and let 

 the cells be not only in contact, but become attached at the points 

 of contact. Then instead of each cell expanding, so as to encroach 

 on and fill up the intercellular spaces, let each cell tend to contract 

 or shrivel up, by the withdrawal of fluid from its interior. The 



ibid. XII, p. 303, 1863; Jeffries Wyman. Pr. Anier. Acad, of Arts and 8c. vir, pp. 

 68-83, 1868; Chauncey Wright, ibid, iv, p. 432, 1860. 



