i02 SEEDS AND SEEDLINGS 



agent. Examples of this kind are the violet, witch-hazel, 

 and touch-me-not, whose capsules dehisce with a little 

 explosion and shoot out the seeds as if they were fairy 

 mortars. It is worth while to gather a bough of witch-hazel 

 in winter and keep it in the schoolroom to watch the explo- 

 sions. In other cases, the carpels curl upwards with a 

 sudden jerk, as in some of the geranium family, or twist 

 themselves into a spiral, like the valves of the rabbit pea 

 ( Vicia americana), thus acting as a spring to eject the 

 seeds. 



136. Animal Agency. — Examples of adaptation for dis- 

 persal by means of animals were given in Section 117, but 

 by far the most active agent in the dissemination of both 

 fruits and seeds is man. This is the frequent result of 

 intention on his part, in the introduction and cultivation of 

 new grains, fruits, and vegetables, and he works to the same 

 end unconsciously and often to his great detriment by the 

 transportation of the bulbs or seeds of pernicious weeds in 

 the dirt clinging to hoes and plowshares, and the mixture 

 of impurities with his crop seeds through ignorance, care- 

 lessness, or unavoidable causes. This mode of dispersal, 

 however, is purely artificial, and except in the case of a 

 few weeds that have adjusted themselves to the conditions 

 of cultivation, is not correlated with any special adaptations 

 in the plants themselves, many of our most widely distrib- 

 uted weeds, such as the rib grass, or common plantain, the 

 mayweed and the narrow-leaved sneezeweed, possessing 

 very imperfect natural means of dispersal. 



PRACTICAL QUESTIONS 



1. Name the ten most troublesome weeds of your neighborhood. 



2. What natural means of dispersal have they? 



3. Which of them seem to owe their propagation to man? 



4. Are there any tumble weeds in your neighborhood? 



5. Should you expect to find such weeds abundant in a hilly or a 

 very woody country? 



6. What situations are best fitted for their propagation ? 



7. Make a list of all the seeds you can think of that are adapted to 

 dispersion by the wind ; by water ; by animals. 



