82 



Vegetable PJujsiology. 



Fig. 9. 



department of physiology, that those plants, the Fungi, growing 

 naturally under conditions as to heat and light analogous to those 

 by which we produce the unnaturally luxuriant and succulent 

 growtlis in such vegetables as seakale, celery, &c,, are charac- 

 terized perhaps most strongly by the fact that they are nourished 

 exclusively upon dead or living organic matters. Let us examine 

 the facts that have been satisfactorily ascertained with regard to 

 the composition, growth, and general vital phenomena of one of 

 the Fungi. 



The Yeast fungus, forming, as collected in masses, the substance 

 known as yeast, consists of a microscopic sac or cell, such as we 

 have already described. The individual plant is a globular 

 vesicle, the membrane of which is composed (Mulder) of a " sub- 

 stance nearly approaching to cellulose in 

 its properties and composition;" in fact, 

 differs in no important respect from the 

 substance forming the cell-membranes 

 and hard parts of plants in general. This 

 membranous sac, however, contains a 

 substance of different composition " re- 

 lated to proteine " (Mulder). By the 

 microscope we ascertain that this internal 

 substance represents the ■protoplasm men- 

 tioned as filling the cell-cavity in Pro- 

 tococcus, and that it exhibits a dense layer 

 where in contact with the cell-membrane, 

 exactly corresponding to the " formative 

 layer" above described. In short, the yeast- 

 fungus is simply a Protococcus-cell, with- 

 out the green colouring matter. The yeast-fungus grows by new 

 vesicles budding out from the surface, into wl;ich a portion of the 

 proteinous or nitrogenous contents is simultaneously transferred, 

 and then the communication is cut off by secretive formation 

 of cellular membrane by the "formative layer." Such budding 

 takes place very rapidly and at various points of the surface when 

 the yeast-plant is well supplied with appropriate food, and what 

 is called fermentation in beer, &c., depends eniirchj upon such 

 multiplication of th.e cells of yeast, the alterations in the fer- 

 menting liquid resulting from the chemical changes connected 

 with the " digestion " or " respiration " of the plants. Now we 

 know from microscopic observation that the formative layer is 

 the prime agent in tlie processes of development and growth of 

 cells as above explained in Frotococcus and the filamentous Con- 

 ferva^, and it is the same here. We know on the authority of 

 Mulder and other chemists, that fermentation will not go on 

 unless a certain supply of nitrogenous food is afforded to the 



(|mj|; 



Group of cells constituting tlie 

 vegetating fonn of the ' Yeast- 

 plant,' magnified 800 diameters. 

 a, groups growing by budding 

 of ibe tells ; 6, cells (the lower 

 budding) treated with tincture 

 of iodine, which coagulales and 

 contracts the protoplasm, leav- 

 ing the outer wall free. 



