96 BOTAJSIT. 



plants living upon and getting their food from living 

 plants) grow in darkness, feeding upon the inner tissues of 

 their hosts (supporting plants) where little or no light 

 penetrates. 



170. — It has been shown by experiment that light some- 

 what retards the growth of certain cells. A shoot grown 

 in darkness or deficient light is always longer than one 

 grown in strong light; but, on the contrary, the leaves on 

 such stems are small and poorly developed. Even in the 

 daily growth of plants the rate during the day is less than 

 during the night. This has been called by Vines the 

 "tonic influence of light." Here we must note that while 

 the stem grows more rapidly in darkness, the leaves grow 

 less rapidly, and in complete darkness remain very small. 



Practical Studies. — (a) Place a plant in the light for a few hours, 

 and then examine the tissues of its leaves, testing by iodine for 

 starch. Place a similar plant in total darkness for 10 to 12 hours and 

 make a similar test. 



(ft) Place a fresh white potato in the sunlight for a few days, and 

 examine thin sections of its tissues for chlorophyll. 



(c) Put a green plant in complete darkness for a few days, and note 

 the disappearance of its chlorophyll. 



(d) Examine a well-blanched leaf of celery ; only leucoplasts will 

 be found. 



(fi) Examine the white, red, blue, purple or yellow petals of 

 flowers ; no chlorophyll will be found, although the flowers may have 

 been in full sunlight. 



(/) Examine the tissues of toadstools and other fungi, (a) grown 

 in darkness and (6) in the light ; no chlorophyll will be found in 

 either. 



(g) Make sections of the stems of dodder (Cuscuta), and note the 

 absence of chlorophyll. 



(/j) Look for moulds and other fungi in dark cellars, as examples 

 of saprophytic plants which have grown without the direct aid of 

 light. 



(i) Cover the end (30 to 40 centimeters) of a cucumber-plant, bear- 

 ing young flower-buds, with a tight box, so as to exclude all light. 

 Notice that the flowers develop perfectly as to size and color although 



