THE PHYSIOLOGY OF PLANTS. 81 



(chemically changed), in which form it can be transported to 

 all parts of the plant. If a portion of the leaf is covered, 

 preventing exposure to sunlight, in that part no starch is 

 formed. 



Cut a cork stopper into layers one quarter of an inch thick. Place one 

 piece on the upper surface of a vigorously growing leaf (for example, Tro- 

 pceolum — "Nasturtium") and another piece on the lower surface directly 

 opposite the first. Thrust two pins through the pieces to keep them in this 

 position. After they have remained twenty-four hours remove them (in the 

 afternoon), sever the leaf from the plant, and put it in boiling water for three 

 or four minutes, to kill the protoplasm and to swell the starch grains. Now 

 remove the chlorophyll by immersing the leaf in alcohol for a day or two 

 and finally plunge it into a solution of iodine (made as directed on page 67). 

 After a short time it will become a deep blue, except over the previously 

 darkened area, where consequently no starch was formed. 



14. The exact function of the chlorophyll itself is not knovpn, 

 but " it is probable that it absorbs certain rays of light and 

 thus enables the protoplasm with which it is intimately con- 

 nected to avail itself of the radiant energy of the sun's rays 

 for the construction of organic substances from carbonic diox- 

 ide and water." (Vines.) The formation of the nitrogenous 

 materials in the plant is not yet clearly understood. It is not 

 however a photo-synthetic process, that is, it is not dependent 

 on the presence of chlorophyll, and apparently takes place at 

 night as well as day. All the transformations or changes that 

 take place in the food materials after the photo-synthetic pro- 

 cess is completed are designated by the term metabolism. 

 These metabolic changes may be regarded as the digestion, 

 and the final conversion into vegetable fabric (tissues) may 

 be called assimilation. These two processes correspond to 

 those in animal physiology which are designated by the same 

 terms. 



15. A small number of plants can capture insects, which in 

 part at least they consume as food. Of such insectivorous 

 plants a common example is the Sundew (Drosera), which 

 grows in bogs and wet places. The radical leaves are fur- 

 nished with stalked glands, wliose glistening secretion impris- 

 ons flies which alight thereon. The flies soon die and are 



