THE CELL. 41 



forms in this section. By way of completion we will add some 

 general remarks on cell-forms and their names, since the expressions 

 and terms concerned pertain to the most valuable language-treasure 

 of scientific botany. 



The terms parenchyma and prosenchyma have long been es- 

 tablished. They do not refer to the mode of growth, but simply 

 to the form of the cells. The term parenchyma is used to 

 designate: 1, all isodiametric and tabular cells (hence all spherical, 

 cubical, parallelepipedal, and polygonal cells) ; 2, all elongated 

 cells having blunt ends (hence all elongated cells with exactly or 

 .approximately rectangular ends). All elongated cells with pointed 

 or sharp endings (hence conical, one or both ends blade-like) 

 are prosenchymatous. The most important representatives of 

 prosencfiyma are the mechanical cells (skeleton-cells) which were 

 named " sterome-cells " or " stereids " by SCHWENDENEK. 



With HABERLANDT we use the term sclerenchyma * to designate 

 considerably thickened non-prosencTiymatous elements which occur 

 isolated or in groups in various tissues exclusive of vascular bundles; 

 therefore in the outer cortex, pith (medulla), etc. Two sclerenchyma- 

 -cells in cross-section are shown in Fig. 22. 



A fitting introduction to a brief consideration of the origin of 

 new cells is the statement of a fact which sometimes causes difficul- 

 ties to the beginner in phytotomy ; it is that every 

 cell has a membrane. This statement holds good 

 for every tissue-structure. On maceration (care- 

 fully boiling, for example, a particle of wood in 

 sulphuric acid and calcium chloride) the tissue 

 separates into its individual elements. Even an 

 immeasurably thin wall between two cells of a Fm * 22 ' 

 mature tissue is thereby split and shown to be double. The macer- 

 ating liquid has dissolved the cementing material. This leads 

 us to the so-called " primary membrane" or " middle lamella," which 

 is, however, not wholly identical with the intercellular cement. 

 The prominent, not immeasurably thin, middle lamella of woody 

 cells differs from the remaining membrane in having a different 

 refractive index. Solubility in the macerating mixture is therefore 



J DE BARY (Comp. Anatomy) designates typical mechanical cells as " scleren- 

 chyma fibres"; hence the terminology here introduced differs markedly from 

 that of de Bary. 



