42 



BOTANY 



PAKT I 



rule the thickening of a partition wall is effected by the two adjoining 

 protoplasts depositing new lamellae on both sides of the original thin 

 septum (Figs. 38, 40, 62). The thickening may thus be equal or 

 unequal on the two sides and each protoplast comes to have its own 

 surrounding layers. The common middle region of the wall is called 

 the middle lamella (Fig. 38 m). It is as a rule very thin, only widen- 

 ing out somewhat at the angles where several walls meet (Fig. 71 Cm*), 

 and consists mainly of pectic substances containing calcium ; it is 

 relatively easily dissolved. In lignified and suberised tissues the 

 middle lamella is also lignified. 



In soft tissues even boiling in water may swell the middle lamella and so 

 separate the cells (e.g. many kinds of potatoes). In ripe fruits this separation 



FIG. 38. Strongly thickened cell from the pith 

 of Clematis vitalba. m, Middle lamella ; i, inter- 

 cellular space ; t, pit ; w, pitted cell wall in 

 surface view, (x 300. After SCHENCK.) 



FIG. 39. Cells from the endosperm 

 of Ornithogaluin umbellatum. m, 

 Pits in surface view ; p, closing 

 membrane ; n, nucleus, (x 240. 

 After STRASBURGER.) 



occurs naturally. Treatment with Schulze's macerating fluid (potassium chlorate 

 and nitric acid) or with concentrated solution of ammonia will separate other cells 

 by destroying the middle lamella. The macerating fluid will thus isolate the 

 elements of wood. There are also certain Bacteria which ferment pectic substances 

 and thus bring about the separation of the cells ; in this way the mechanical cells 

 of Flax are isolated in the process of retting. 



The thickening layers are distinguishable from the middle lamella 

 both by their optical and chemical properties. Since they usually 

 lie equally on both sides of the middle lamella the whole wall acquires 

 a more or less symmetrical construction (Figs. 38-40, 41, 62) which 

 extends to the pitting. Three distinct layers can frequently be 

 distinguished in strongly thickened cell walls, such as those of the 

 wood, a primary, a secondary, and a tertiary thickening layer; these 



