SUPPLEMENT 



The teacher must make sure that the root marking experiment is 

 thoroughly comprehended. The marks are at first equidistant, but 

 the mark on the tip is carried further and further on, while the others 

 remain exactly as they were. If roots were to grow throughout their 

 length through the unyielding soil they would necessarily become 

 much distorted. The root-tip is protected by a little cap that is 

 renewed as fast, as it is worn away by friction. The remarkable 

 properties of root-tips are treated at length by Darwin in his " Power 

 of Movement in Plants." In his enthusiasm over the results of his 

 experiments with seedlings he says : " It is hardly an exaggeration to 

 say that the tip of the radicle, thus endowed, and having the power to 

 direct the movements of the adjoining parts, acts like the brain of 

 one of the lower animals ; the brain being seated within the anterior 

 end of the body, receiving impressions from the sense-organs, and 

 directing the several movements." Some German critics consider 

 this an extravagant statement, but one of them, Sachs, in his 

 " Physiology of Plants," explicitly states that the end of the root is 

 sensitive to pressure and to moist surfaces, and that as a result the 

 root curves so as to avoid the obstacle and reach the moist 

 substratum. 



The tendency of the main root to grow directly downward can be 

 easily shown by pinning seedlings to cork in the moist air chamber 

 so that light and moisture are everywhere equal. The roots of seed- 

 lings grown on gauze, or on a sponge over water, clearly grow away 

 from the light. Experiments to show the spiral movement and sensi- 

 tiveness to moisture and pressure require more time and apparatus. 



The padding of roots, which protects them against evaporation 

 and mechanical injury, can also be noted here. The natural length of 

 roots will not be very accurately determined from seedlings grown in 

 shallow dishes. Wheat, out of doors, has been known to send down 

 roots to a depth of seven feet, and the sum total of the length of all 

 the roots of an oat plant is sometimes as much as one hundred and 

 fifty feet. Seeds grown in a crowded condition show the power of 

 roots to bind soil together so that it will resist the force of wind or 

 rain. But the main point here is to emphasize the great extent of 

 the absorbing surface furnished by this elaborate root system, that is, 

 its capacity for taking in food. 



Some of the minerals needed by all plants are potassium, sodium, 

 calcium, sulphur, magnesium, phosphorus and iron. The proportions 

 of the minerals required vary with different plants as every agricul- 

 turist knows, but by striking an average, these salts, with necessary 

 gases, can be artificially combined into a. nutritive solution in which 



