10 BOTANY PART i 



the external members of the plant as its organs, taking into considera- 

 tion both their external form and internal structure. 



SECTION I 



CYTOLOGY 



THE CELLS AS THE BASIS OF LIFE 



I. FORM AND SIZE OF CELLS 



As already mentioned, both plants and animals are constructed of 

 elementary parts known as cells. In the case of plants these are 

 microscopically small chambers, the walls of which are formed of a 

 firm membrane. In this respect they differ from animal cells. In 

 the simplest cases the cells are spherical, but more commonly they 

 have the form of small cubes, polyhedra, or prisms, which are 

 associated in large numbers in the multicellular organs of plants. 

 Elongated cells forming fibres or tubes are also of frequent occurrence. 

 These chambers, each of which consists of the 

 cell wall or cell membrane enclosing the cavity 

 or lumen of the cell, are as a rule so small as to 

 be visible only when highly magnified. Their 

 mean diameter is frequently between the hun- 

 dredth and tenth of a millimetre. Owing to 

 this it was long before the existence of cells 

 was recognised. Occasionally cells attain a much 

 greater size. Some sclerenchyma fibres adapted 

 to special functions are 20 cm., while laticiferous 

 [ tubes may be some metres in length. 



bottle-cork, which he de- The most important part of the cell is the 

 scribed as "Schematism protoplast or cell body occupying the cavity en- 



or t6xt/ur6 of cork. Cf. i in , i 11 n i i i . . 



Fig> 58> closed by the cell wall, since this is the living 



portion of the cell. On this account it is now 



natural to think rather of the living protoplast than of its enclosing 

 chamber as the cell ; a cell wall is completely wanting in the case of 

 many "naked cells." In dead cells, it is true, the protoplasts have 

 almost or completely disappeared, and such cells are only empty cell 

 cavities. With the death of their protoplasts these cells need not 

 lose their use to the plant. They are indeed essential in the construc- 

 tion of the more highly organised plants in which dead cells form the 

 water-conducting tracts and contribute to mechanical rigidity. 



It was due to the investigation of the cell walls that cells were recognised first 

 in plants. An English micrographer, ROBERT HOOKE, was the first to notice 



