2G THE ELEMENTARY STRUCTURE OF PLANTS. 



The smaller of these sizes Avould allow of as many as 1728 millions 

 of cells in the compass of a cubic inch ! 



24. Some idea may be formed respecting the rate of their pro- 

 duction, by comparing tlieir average size in a given case with the 

 known amount of growth. Upon a fine day in the spring, many 

 stems shoot up at the rate of three or four inches in twenty-four 

 hours. When the Agave or Century-plant blooms in our conser- 

 vatories, its flower-stalk often grows at the rate of a foot a day ; it 

 is even said to grow with twice that rapidity in the sultry climate to 

 Avhich it is indigenous. In such cases, new cells must be formed at 

 the rate of several millions a day. The rapid growth of Mu.-dirooms 

 has become proverbial. A gigantic species of Puff-ball has been 

 known to attain the size of a large gourd during a single night : 

 in this case the cells of which it is composed are computed to have 

 been developed at the rate of three or four hundred millions per 

 hour. But this rapid increase in size is owing, in great part, to the 

 expansion of cells ali-eady formed. 



25. The Cell as a living Organism. Thus far we have considered 

 only the membrane or permanent wall of the cell, — that which 

 makes up the tissue or fabric of plants, and which remains un- 

 altered, and performs some of its offices even long after life has 

 departed. But we should now regard the cell as a living thing, 

 and consider what the wall encloses, and what operations are 

 effected in it. For the whole life of the plant is that of the cells 

 which compose it ; in them and by them its products ai'e elaborated, 

 and all its A'ital processes carried on. 



26. A young, living, vitally active cell consists, — 1st, of the 

 membrane or permanent wall, already described ; 2d, of a delicate 

 mucilaginous film, lining the wall, called by Mohl the primordial 

 utricle ; 3d, most commonly the centre of the cell, and sometimes 

 the greater piu't of the cavity, is occupied by the nuclevs, a soft 

 solid or gelatinous body ; and 4tli, the space between the nucleus 

 and the lining membrane is filled at first by a viscid liquid, called 

 protoplasm, having an abundance of small granules floating in it. 

 As the cell enlarges by the growth and expansion of its walls, the 

 space between the latter and the nucleus becomes filled with watery 

 sap, leaving the protoplasm merely as a viscid coating of the inside 

 of the primordial utricle, and of the nucleus, if this remains. 



27. The cell-membrane, or proper wall of the cell, is chemically 

 composed of the three elements, carbon, hydrogen, and oxygen, 



