FORMS OF CELL. 



23 



Cellulose is colourless when pure, but is coloured blue 

 when treated with sulphuric acid and iodine. This reaction 

 is known as a test for cellulose. 



During the increase in size of the cell-wall 

 growth is rarely uniform at all points, but 

 certain portions of the surface grow at a 

 much quicker rate than others, which results 

 in the originally spherical cell presenting an 

 irregular outline at maturity. This unequal 

 rate of development is known as local growth. 

 As examples of the result of local growth 

 may be mentioned the stellate or star-shaped 

 cells of the pith of some rushes, the spines 

 and warts on the walls of pollen, and the 

 spores of many ferns, mosses, and funguses. 

 Very much branched cells are to be found 

 mixed with the dense powdery mass of spores 

 in the puff-balls, but the most remarkable 

 forms assumed by single cells are met with in 



Fig. 3-— (x 240)- 



A cell from the hair on ihejllament or stalk of the stamen 

 of Virginian Spiderwort ( Tradescantia virginica). The outside black 

 line is the cell-wall, lined by a thin, unequal layer of protoplasm, from 

 which proceed strands of protoplasm into the sap-cavity ; in the centre 

 of the cell the large spherical nucleus is seen lying in the protoplasm . 

 (From Strasburger.) 



the unicellular or one-celled members of the seaweed family 

 {Alga), where the species of Caulerpa inhabiting tropical 

 seas are often two feet in length, and possess portions 

 resembling root, stem, and leaf respectively. Such gigantic 

 cells are exceptions to the rule previously stated, that cells 

 are usually so minute as to be invisible to the naked eye. 

 The last example illustrates the great amount of differentia- 

 tion and division of labour that a single cell is capable of 



