GKOWTH AND CONSTRUCTION OF PLANTS. 



1.— THEORY OF GROWTH. 



Conditions and Mechanics of Growth. — Influence of Growing Cells on their Environment. 



CONDITIONS AND MECHANICS OF GEOWTH. 



Whoever wishes to germinate seeds must moisten the earth selected as soil, 

 or else must supply water to the seeds in some other way. The seeds absorb 

 the water; the embryo bursts its covering, sends out rootlets into the ground, and 

 its stem and leaves grow up towards the light. The young seedlings must now 

 be diligently watered if they are to flourish and increase in bulk, for they require 

 for their growth an astonishingly large amount of water. Other plant organs 

 which it is desired should grow or be kept in vigorous development are like the 

 seeds, and the suitable watering of cultivated land is, and always has been, one 

 of the fundamental conditions of plant culture. In uncultivated districts the 

 dependence of growth on the water supply appears no less remarkable. Where 

 vegetative activity is brought to a standstill not by the cold of winter, but by 

 heat, the commencement of the rainy season is, each year, the signal for the 

 revival of growth. The amount and duration of the rainfall govern in a most 

 striking way the whole progress of plant development. As soon as the first 

 moisture soaks through the soil, after a long drought, the plants wake up from 

 their lethargy, the dry, sunburnt landscape becomes adorned with vivid green, 

 and the luxuriance of the shoots and leaves arising from the seeds and buds 

 stands in strict proportion to the quantity of water daily supplied to the growing 

 plants. 



Why do plants require these quantities of water? The answer to this question 

 has already been partly given in a previous section of this book, when it was 

 shown how the absolutely necessary mineral food-salts were taken up by means 

 of water; how the water in which the food-salts are dissolved is conveyed by 

 root-pressure and by suction to the place of consumption. But this is certainly 

 not the only significance water has for plants, for it would leave unexplained 

 why growing seedlings which cannot yet absorb mineral food from the earth, and 

 which do not even require it, still consume so much water. It must also be 

 remembered that those chemical processes in vegetable cells in which mineral food- 

 salts are worked up do not yet themselves constitute growth, but only a prepara- 

 tion for growth. Mineral salts play an important rSle in the transformations going 



