68 SCLEREIDES 



as follows: (1) Brachysclcreides, or "stone-cells" which are 

 roughly isodiametric in form and which occur in the fleshy por- 

 tion of the fruits of Pijnis and Cyclonia and in the bark of numer- 

 ous woody dicotyledons; (2) Macrosclereides, or "rod-cells" 

 which are columnar in form and often constitute an outer con- 

 tinuous palisade-like layer in the coat of seeds, especially in the 

 family Leguminosae. Here they are also referred to as "Mal- 

 pigliian Cells"; (3) Osteosdereides, or "bone-cells" which are 

 likewise columnar in form but possess dilated or knob-like ends. 

 Such cells occur within the palisade parenchyma in certain leaves 

 (cf. Haberlandt, p. 160, Fig. 52) ; and (4) Asirosclereides or 

 "branched sclereides," w^hich are highly irregular in form and 

 size with pointed "arms." Astrosclereides are well developed in 

 the leaves of certain dicotyledons (e.g., Thca, Camellia) and also 

 occur in the bark of Abies and Larix and in certain fruits (e.g., 

 Carya). 



The ontogeny of sclereides presents many distinctive and 

 peculiar features. Aside from the macrosclereides, which are 

 traceable in origin to the surface cells or "protoderm" of the 

 integument of the ovule (cf. Zimmerman, 1936), sclereides usu- 

 ally develop by the "secondary sclerosis" of parenchymatous 

 cells (cf. De Bary, pp. 539-544). This curious process involves 

 the marked centripetal increase in thickness of the cell wall, the 

 deposition of lignin within the cellulosic matrix and the produc- 

 tion of the characteristic "pit-canals" or "ramiform pits." The 

 physiological factors which induce these changes in a living par- 

 enchyma cell or cell group in the cortex or the bark of stems or 

 roots are obscure. Tschirch (1889, p. 303) states that in woody 

 dicotyledons, the process of secondary sclerosis occurs to such 

 an extent that sclereides may eventually constitute the major 

 porticm of the bark. A further interesting aspect of the process 

 of secondary sclerosis is exhibited by the development in many 

 stems of the so-called "composite cylinder" formed of both 

 brachysclcreides and bast fibers. In this case, an originally 

 continuous cylinder of bast fibers becomes ruptured at various 

 points as a result of the increase in thickness of the stem. Neigh- 

 boring parenchyma cells then intrude into the gaps, divide and 

 eventuallv become transformed into sclereides thus "repairing" 



