90 



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 swollen-up mucilaginous layers 

 forming a coherent mass. 



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

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



iW^o 



I 



Fig. 89.— a parenchyma cell from the 

 cotyledon of Phaseolus multiflorus isolated 

 by maceration, i, i the rounded corners 

 bounding intercellular spaces ; t, t the walls 

 in contact with neighbouring cells, and pro- 

 vided with pits. 



Fig. go.— Transverse section of a scle- 

 renchymacell from the root-tuber of Z)«A/iVr 

 variabilis (x 800). / the lumen of the Cell ; 

 K pit-canals which penetrate the stratifica- 

 tion ; sp a fissure by which an inner system 

 of 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, espe- 

 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 lamellae, some poor in 

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



