142 BOTANY: PRINCIPLES AND PROBLEMS 



mitotic division, causes most of the obvious increase which we 

 see in the size of plant parts. 



Cell Maturation.- — The new tissue thus formed is very soft and 

 weak, owing to the thin walls of its component cells, and the third 

 stage in growth, maturation, is brought about by the transfer of an 

 abundant supply of food into these newly-formed parts and the 

 consequent construction therefrom of new living substance and of 

 heavier walls until the cells have reached the normal, mature 

 condition characteristic of the particular tissue of which they 

 form a part. 



Growing-points and Their Function. — Such, in brief, is the 

 history of the production of new cells by which the growth of the 

 plant body takes place. It is evident, however, that in mature 

 plant tissues, where the cells are surrounded by thick and firm 

 walls, cell division is no longer possible. Such tissues are thus 

 really locked within their own walls and can grow no further. 

 Organs like the leaf and flower, which rapidly attain a rather 

 definite size beyond which growth no longer takes place, do not 

 present this problem, for here the organ develops from a small 

 mass of growing tissue, enlarges rapidly throughout its whole 

 extent and reaches maturity in all its parts at once, when growth 

 stops. In such organs as the root and stem, however, where 

 growth continues more or less indefinitely and where the great 

 bulk of the tissues are necessarily mature and functioning, there 

 must obviously be some way of insuring the continued production 

 of new cells. This is accomplished through the activity of 

 growing-points or meristems, which are merely groups of cells 

 remaining in an embryonic and undifferentiated condition, 

 thin-walled and packed with protoplasm. These groups of 

 permanently "young" cells occupy regions where growth is to 

 take place, as at the tip of the root or stem or at the cambium. 

 Such a growing-point may long remain dormant, but when it 

 becomes active, cell division begins again within it. The newly 

 formed cells which lie next to the already mature tissue now 

 undergo enlargement and become themselves mature. This 

 process does not affect all of the cells of the growing-point, 

 however, for the portion away from the maturing cells still 

 remains undifferentiated and continues to serve as a manu- 

 factory of young cells which are to be added to the tissue. The 

 growing-point is thus a rather small and inconspicuous group of 

 cells, not increasing in bulk itself, but carried progressively out- 



