284 TEXTBOOK OF PLANT PHYSIOLOGY 



perature, light, supply of nutrient substances, etc. Consequently, 

 these formulae represent the process of growth only approximately, 

 Robertson's formula having the advantage that it attempts to 

 cover the whole period. 1 



The causes which retard and check growth are not fully under- 

 stood at present. Here one must consider the general problem of 

 senescence of an organism, which is still unsolved. 2 



86. Growth of Individual Cells and Its Mechanism. The 

 Three Stages of Growth: Embryonic Development, Expansion, 

 and Internal Differentiation. — Fundamentally, the growth of 

 plants and their organs consists of the multiplication and growth 

 of the cells composing them. Therefore, in order to understand the 

 laws of growth, the alterations occurring during growth in the cells 

 must be studied. Not all the cells of a plant, however, participate 

 in its growth. The greater part of the plant consists of mature 

 cells. Only in certain regions are there found meristematic tissues 

 consisting of growing cells, or tissues capable of growth and multi- 

 plication. These are the so-called growing points, forming the 

 tips of stems and roots. 



A detailed description of the structure of embryonic regions and 

 meristematic tissues in general may be found in textbooks of 

 plant anatomy. Here it will be indicated only that their cells 

 are comparatively small, have thin walls, and are completely 

 filled with protoplasm containing a large centrally located nucleus. 

 Growth of meristematic tissues proceeds in the following manner: 

 The amount of protoplasm in the cells gradually increases, resulting 

 in an increase in size of the cells themselves. Thereupon, division 

 of the nucleus and of the protoplasm takes place and the newly 

 formed cells are separated from each other by new walls. The 

 accumulation of living substance begins anew, followed by a new 

 division of cells. Consequently, in the meristematic region the 

 fundamental process is an increase in the number of cells and in the 

 general mass of living matter. 



In organs with uninterrupted growth, such as roots or young 

 stems, the total mass of meristematic tissue remains constant, in 

 spite of the continuous division of its cells. This is due to the fact 



1 Of only certain kinds of plants, primarily those with a discontinuous 

 type of growth, such as the sunflower, wheat, etc. 



2 Correlation between vegetative and reproductive functions often deter- 

 mines the character and rate of growth. 



