108 PLANT BIOLOGY 



116. Bulbs. Still another method of propagating plants is 

 by means of bulbs, which, in the onion, for example, consist of a short, 

 thickened underground stem, to which are attached many layers 

 of thickened parts of leaves known as bulb-scales. Frequently, as 

 in the tulip and hyacinth, after the food stored in the bulb has been 

 used in the early spring to develop stem, leaves, and flowers, the 

 nutritive organs store away food for another season by producing 

 new bulbs close to the old one. 



117. Home bulb culture. (Optional.) 



Few plants are easier to cultivate or give greater satisfaction, 

 especially in winter, than those that grow from bulbs. Secure a fe\\ 

 tulip, hyacinth, or narcissus bulbs and bury them in pots of rich 

 earth. Water them well and put them in a dark, cool place for four 

 to six weeks, until the roots appear through the opening at the bot- 

 tom of the pot. Then put them in a warm, sunny place, keep them 

 well watered, and the flowers will appear in a few weeks. 



III. CONDITIONS THAT ARE ESSENTIAL FOR THE GROWTH 



OF PLANTS 



118. The five essential conditions for plant growth are the 

 following: (1) moisture, (2) favorable temperature, (3) air, 

 (4) light, (5) food. We have already shown the necessity of 

 air for the germination of seeds (79) and for the liberation of 

 energy (76). The use of the food stored in the corn grain 

 for the corn embryo was also demonstrated in 112. We have 

 also proved that green plants do not manufacture carbohy- 

 drates in the absence of sunlight (30) . We can likewise show 

 experimentally the relation of moisture and temperature to 

 the germination of seeds and to the growth of plants, 



119. Relation of moisture to germination and growth. Labo- 

 ratory Study No. 56. (Optional.) Suggested as home work. 



Secure three tumblers of same size (tin covered jelly-tumblers are 

 very satisfactory). In the bottom of each put a piece of 



