HOW PLANTS GROW. 223 



nucleus, or by ahsorption of it, and re-formation of as many- 

 fresh nuclei as there are daughter-cells to be produced. After 

 this point is reached, there occur two different conditions in the 

 process of cell-multiplication, known as " Free Cell-formation " 

 and "Cell-division." 



Free Cell-formation. In this, the protoplasm masses of the 

 mother-cell gather round the previously-formed nuclei, and so form 

 new cells enclosed within the mother-cell, which still for some time 

 retains life, so that only part of its protoplasm goes to make the 

 daughter-cells. The cell-wall is in most cases formed while the 

 new cells are still inside the mother-cell, as in the " embryo-sac " 

 (of which presently), and in pollen-grains. In some fewer cases, 

 it is formed after the escape of the new cells, as in the swarm- 

 spores of some fungi and lichens. 



Cell-division. This is by far the commonest method of multi- 

 plication. Only this obtains in all processes of ordinary growth, 

 such as tissue-forming, etc. Here the protoplasm of the mother- 

 cell divides into many portions by repeated bi-partition, following 

 a similar process affecting the nucleus. These fill up the entire 

 space inside the mother-cell, leaving room only for the cell-walls to 

 form, so that in this case all the maternal protoplasm goes to 

 make the daughter-cells. Sometimes the formation of cell-walls 

 goes on pari passu with division of the protoplasm, as in filamen- 

 tous AlgcB and the pollen-grains of many dicotyledons. In others 

 it is sudden, as in the pollen-grains of most monocotyledons. 



In tissue-formation this division is repeated again and again, 

 the resulting cells increasing to the size of the parent ones, and 

 again dividing. Meantime, there also ensues an increase in thick- 

 ness, although, for the most part, this does not go on until the cell 

 has attained its full size, when the secondary layers, already des- 

 cribed, come about. 



Not only is the number of cells thus increased by cell-division, 

 but seeing that they retain connection one with the other, we get 

 the various cellular structures, finding their completest develop- 

 ment in the adult plant. 



We cannot now say more about the cell and its growth. The 

 question, "How do plants grow?'' is easily replied to in cases of 

 one*celled plants, even though these may be of considerable size, 



