6 



PRINCIPLES OF PLANT CULTUBE 



11. Health and disease. — A plant or animal Is said 

 to be in health when all its organs (parts) are capable of 

 performing their normal functions. An organ incapable 

 of doing this, or the being possessing such an organ, is said 

 to be diseased. 



12. The cellular structure of living beings. — A bit of 

 vegetable or animal substance, examined under a micro- 

 scope of moderately high 

 power, is seen to be made 

 up of numerous little 

 sacks or cavities, more or 

 less clearly defined, called 

 cells. Cells from differ- 

 ent beings, or from differ- 

 ent parts of the same 

 being, may vary much in 

 form and size, but they 

 are seldom large enough 

 to be seen without mag- 

 nifying power. Some of 



_ „ „ „ , , , , ■ the lowest plants and 



riG. a. — Cells of an apple leaf m . , . „ . , 



a section from its upper to its lower animals COnSlSt ot smgle 

 surface. Higiily magnified. The 

 spaces marked / are cavities be- 

 tween the cells. 



Snsssc) 



cells (Fig. 1). Some of 

 the lower plants consist 

 of a single row of cells 

 united at the end (Fig. 2). The higher plants and ani- 

 mals are made up of many cells united, and in these, the 

 cells assume different forms and properties in the different 

 organs (Fig. 3). In some cases the united cells may be 

 readily separated from one another, which shows each 

 cell to be more or less an independent structure. As a 

 rule, each cell is surrounded by its own closed cell- 

 wall. 



