9° 



LECTURE VI. 



metamorphosis, we may consider the formation of Bassorin, and finally that of soluble 

 gum-arabic. This mucilaginous change of the cell-walls may, under certain cir-; 

 cumstances, occur as a diseased condition, as in the gum-formation of Plums and 

 Cherries ; in other cases, on the other hand, especially when the mucilaginous change 

 is associated with only a slight alteration of the original cellulose, it appears as a 

 normal change serving definite purposes of life. Numerous seeds and dry indehiscent- 

 fruits possess within their epidermis mucilaginous layers of cell-membrane, which, on 

 being moistened with water, break through the cuticle and surround the seed or the 

 fruit as a gelatinous envelope. This is the case for example with the seeds of the 

 Quince, the Flax, Plantago psyllium, &c. ; if a large quantity of the small grains 

 is moistened with water, a viscid paste is formed, the swoUen-up mucilaginous layers 

 forming a coherent mass. 



In the economy of the Algge and many Fungi (especially certain Gastero- 

 mycetes), and some Lichens (e. g. the Collemacese), the mucilaginous change of the 



FIG. 89.— A parenchyma cell ftom the 

 cotyledon of Phaseolits mtiltzjiorus isolated 

 by maceration, i, i the rounded corners 

 bounding intercellular spaces;^, /the walls 

 in contact with neighbouring cells, and pro. 

 vided with pits. 



Fig. 90. — Transverse section of a sde- 

 renchyma cell from the root-tuber of Dahlia 

 ■variabilis (x 800). / the lumen of the cell ; 

 K pit-canals which penetrate the stratifica- 

 tion : sp a fissure by which an inner system 

 Df layers has become separated. 



cell-walls plays a very prominent part ; so much so, indeed, that the form and mode 

 of life of such plants are to a certain extent determined by it. 



These chemical alterations of the cell-walls, here briefly described, need by 

 no means always invade the entire thickness of a cell-wall. It is often only 

 definite layers or shells of it which are affected by the changes named. The 

 suberisation or cuticularisation frequently takes place only at the exterior, espc 

 cially in the case of isolated cells; whereas lignification tends to affect the 

 middle layers of strongly-thickened cell-walls; while the formation of mucilage may 

 affect either the middle lamella or any other layer of the cell-wall. 



The extremely thin walls of young vigorously growing cells usually appear quite 

 homogeneous, even with strong magnifying powers. In thicker, and especially in very 

 thick cell-walls, on the other hand, a concentric stratification is recognised on the trans- 

 verse section, to which a corresponding marking is again found also in the longitudinal 

 section. With particularly clear objects it is perceived that this concentric strati- 

 fication of thickened cell-walls depends upon the alternation of lamellee, some poor in 

 water and hard, others richer in water and soft, which together make up the cell-wall. 



