Exercise VIII 



SCLEREIDES 



I. Introduction. — From tlie standpoint of function, two gen- 

 eral types of strengthening or "mechanical" tissues are conven- 

 tionally distinguished, viz.: (1) collenchyma, which is composed 

 of living cells and which is the first strengthening tissue to appear 

 in the development of the stem and leaf, and (2) sclerenchyma 

 which is made up of thick-walled cells, usually lacking a proto- 

 plast at maturity and which represents the "permanent" and 

 more important of the supporting tissues in the older portions of 

 the plant body. From a morphological standpoint, many authors 

 (e.g., Eames and INIacDaniels, Ilayward) adopt the viewpoint of 

 De Bary (pp. 126-184) and recognize two main "forms" of 

 sclerenchyma; namely, "stone cells" which are short, more or 

 less isodiametric elements and fihers which are prosenchymatous 

 cells, often extremely long and with pointed or oblique ends. 

 Despite the convenience of this morphological subdivision of 

 sclerenchyma it seems hardly justified when the numerous struc- 

 tural and ontogenetic differences between so-called stone cells 

 and fibers are clearly understood. For this reason, the term 

 "sclerenchyma" appears vague and highly abstract and, as will 

 appear in the resume to follow, has been used to designate cell 

 types which are definitely unrelated in wall structure and method 

 of development (cf. Ilaberlandt, p. 721, note 92). 



The expression "sclereide" was originally proposed by 

 Tschirch (1885, p. 308) for the variously formed thick-walled 

 cells which occur so commonly in the bark of woody seed plants, 

 in the hard shell of fruits, and in seed coats. In contrast to 

 typical "bast fibers," the thick walls of sclereides often appear 

 yellow in color, are usually highly lignified and possess tubular 

 pits which may branch in a complex fashion. On the basis of 

 form and structure, Tschirch (1889, pp. 301-302) distinguished 

 four principal types of sclereides which he named and described 



67 



