n8 SEEDS AND SEEDLINGS 



direction of organs that seems to defy all law. Heat, un- 

 less excessive, generally stimulates growth ; contact some- 

 times simulates it, causing the stem to curve away from the 

 disturbing object, and sometimes retards it, causing the stem 

 to curve towards the object of contact by growing more 

 rapidly on the opposite side, as in the stems of twining 

 vines. Light stimulates- nutrition, but generally retards 

 growth. The heliotropic movements of plants (Sections 

 54-57) are effected in this way ; the growth being checked 

 on that side, the plant bends toward the light 



163. Internal Forces of the Plant. Another important 

 factor exists in the internal constitution of the plant itself. 

 Place a segment of prickly pear (Opuntia) or other cactus, 

 tip downward in the soil ; roots will develop with great 

 difficulty : because the natural forces of the plant tend to 

 carry the root forming material to the base, and it takes 

 time for the external factors of dampness, moisture, and 

 gravitation to overcome this inherent tendency. Place two 

 leafy twigs of some herbaceous plant, one in its natural 

 position, the other bottom upwards, in a vase of water, 

 and notice the difference in the wilting of the leaves, due 

 to a physiological tendency in the conducting cells to carry 

 the crude sap toward the apex. 



PRACTICAL QUESTIONS 



1. Why do stems of corn, wheat, rye, etc., straighten themselves 

 after being prostrated by the wind ? (162.) 



2. Can a plant grow and lose weight at the same time ? (155.) 



3. Do plants grow most rapidly in the daytime, or at night? (162.) 

 4.^ Reconcile this with the fact that green plants will finally die if 



deprived of light. 



5. Which would be richer in nourishment, hay cut in the evening or 

 in the morning ? Why ? (24, 25, 26, 162.) 



6. Which grows more rapidly, a young shoot or an old one ? 



7- Which, as a general thing, are the more rapid growers, annuals 

 or perennials ? Herbaceous or woody-stemmed plants ? 



8. Name some of the most rapid growers you know ? 



9. Of what advantage is this habit to them ? 



1 Sachs, " Physiology of Plants." 



