CHAPTER XIII. 



GROWTH. . 



176. By growth is usually meant an increase in the bulk of 

 the plant accompanied generally by an increase in plant sub- 

 stance. Among the lower plants growth is easily studied in 

 some of the fungi. 



177. Growth in mucor. Some of the gonidia (often called 

 spores) may be sown in nutrient gelatine or agar, or even in 

 prune juice. If the culture has been placed in a warm room, in 

 the course of 24 hours, or even less, the preparation will be ready 

 for study. 



178. Form of the gonidia. It will be instructive if we first 

 examine some of the gonidia which have not been sown in the cul- 

 ture medium. We should note their rounded or globose form, as 

 well as their markings if they belong to one of the species with 

 spiny walls. Particularly should we note the size, and if possible 

 measure them with the micrometer, though this would not be 

 absolutely necessary for a comparison, if the comparison can be 

 made immediately. Now examine some of the gonidia which 

 were sown in the nutrient medium. If they have not already 

 germinated we will note at once that they are much larger than 

 those which have not been immersed in a moist medium. 



179. The gonidia absorb water and increase in size before 

 germinating. From our study of the absorption of water or 

 watery solutions of nutriment by living cells, we will easily un- 

 derstand the cause of this enlargement of the gonidium of the 

 mucor when surrounded by the moist nutrient medium. The 

 cell-sap in the spore takes up more water than it loses by diffu- 



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