CHAPTER IV 

 PROTECTION FROM INJURIES AND LOSS OF WATER 



The simplest unicellular plants cover themselves with a thin 

 cellulose cell-wall. This is harder and tougher than the proto- 

 plast and preserves its form and protects it from injuries; but 

 since it must be of a nature to allow the passage inward of water 

 and solutes it cannot be efficient in keeping the protoplasts from 

 drying up. It is found that if a cell-wall will allow water to pass 

 in it will also let it out; and while the plasmatic membrane is 

 able to retain osmotic substances in solution in the water of the 

 cell-sap it is unable to prevent the loss of the water itself. The 

 unicellular plants, or those consisting of a few cells only, have 

 found that they can inhabit only wet and moist places, or else be 

 able to exist for longer or shorter periods in a condition of extreme 

 desiccation. As soon as plants in the course of their evolution 

 increased in size and complexity and began to burrow into the earth 

 for its treasures of water and minerals, and, at the same time, to 

 rise into the air and light, where they could appropriate other 

 raw materials and the sun's energy, they found themselves exposed 

 to danger of destruction from mechanical injuries and loss of 

 water; and they found in working out the problem, which such 

 plants have had to solve, of division of labor among different sets 

 of cells composing the body, that it was necessary to assign one 

 or more exterior tissues to the function of protection. 



Two tissues, the epidermis and the cork, have accordingly 

 been evolved for the specific purpose of protection, and two others, 

 the collenchyma and superficial sclerenchyma cells and fibers 

 which belong to the skeleton of the plant, also give protection by 

 reason of their hardness, toughness, and tensile strength. With- 

 out such safeguards large terrestrial plants standing ready at 



6t 



