4§ ANCIENT PLANTS 



sibility of a beginning of specialization in the different 

 parts of the cell. The single living cell is equally acted 

 on from all sides, and in itself it must perform all the life 

 functions; but where four lie together, each of the four 

 cells is no longer equally acted on from all sides. This 

 shows clearly in the diagram of a divided cell given in 

 ficr. 1 8. Here it is obvious that one side of each of the 

 four cells, viz. that named a in the diagram, is on the 

 outside and in direct contact with the water and external 

 things ; but walls b and c touch only the corresponding 

 walls of the neighbouring cells. Through 

 walls b and c no food and water can enter 

 directly, but at the same time they are 

 protected from injury and external stimu- 

 lus. Hence, in this group of four cells 

 there is a slight differentiation of the 

 sides of the cells. If now we imagine 

 Fig. i8.— Diagram that cach of the four cells, still remaining 

 "dSoTulSIt iri contact, divides once more into four 

 cells. Walls a are e.x- members, each of which reaches mature 

 temai, and walls * and gi2e while all remain together, then we 



c in contact with each , ^ . ° ,, - 



other. have a group oi sixteen cells, some ol 



which will be entirely inside, and some 

 of which will have walls exposed to the environment. 



If the cells of the group all divide again, in the 

 manner shown the mass will become more than one cell 

 thick, and the inner cells will be more completely dif- 

 ferentiated, for they will be entirely cut off from the 

 outside and all direct contact with water and food 

 materials, and will depend on what the outer cells 

 transmit to them. The outer cells will become special- 

 ized for protection and also for the absorption of the 

 water and salts and air for the whole mass. From 

 such a plastic group of green cells it is probable that 

 the higher and increasingly complex forms of plants 

 have evolved. There are still living plants which cor- 

 respond with the groups of four, sixteen, &c., cells just 

 now theoretically stipulated. 



