CELL-CONTENTS AND FORMS OF CELLS. 157 



lentils, 2.75; maple sap, 2 to 4; oats, 0.51 to 5.27; onions, 0.44 

 to 14.02; rye, 0.39 to 9.46; sorghum juice, 8.60 to 14.70; sugar- 

 cane juice, 16.00 to 18. 10 ; spinach, 0.06 to 6.66 ; turnip ( Swedish ) , 

 5.05 to 9.67; sweet potato, 0.32 to 8.42; tomato, 2.53 to 3.86; 

 vanilla, 7.07 to 9.10; wheat, 0.58 to 5.12. 



Honey-dew is a pathological sugar formed as a result of 

 the stings of insects (Aphides and CoccidccX) on the leaves of 

 certain trees. There are a number of trees the leaves of which, 

 during the summer time, are covered with a thin layer of sugar 

 solution. Among these may be mentioned the linden, tulip poplar, 

 and chestnut. Honey-dew may also be formed, according to 

 Bonnier, without the assistance of aphides, and may be seen oozing 

 out of the stomata. It may be formed in such quantities that 

 it may drip from trees, as in the so-called rain trees of the 

 Tropics (see Pfeffer, '' Physiology of Plants "). 



The Origin and Formation of Carbohydrates. — The first 

 visible product of photosynthesis is starch, and this is sometimes 

 called photosynthetic starch. Investigations during recent years 

 seem to indicate that grape-sugar or dextrose is the basal photo- 

 synthate, and that from this starch is later formed in the plastid. 

 This sugar is called photosynthetic grape-sugar to distinguish it 

 from the grape-sugar found in the cell-sap of the grape, raisins, 

 figs, etc. There is no question but that in the plastids starch 

 is readily formed from glucose, and, vice versa, that the starch 

 in the plastids is readily changed through the agency of the 

 ferment, amylase, into grape-sugar. 



There are four factors necessary for the formation of a i)hoto- 

 synthetic carbohydrate (starch or glucose) by the chloroplastids : 

 ( I ) Light ; and in this condition it is the energy of the red and 

 blue rays of sunlight which are necessary to bring about the 

 synthesis. (2) Carbon dioxide. This compound must be present 

 in about the normal proportions that we find it in the air, namely, 

 3 parts in 10,000. (3) Water is essential, and this is always pres- 

 ent in living cells. It is by the dissociation of the CO._. and ILO 

 and rearrangement of the atoms that carbohydrates are formed, 

 being either starch (Q-HjoO,) or glucose (Celli.O,), with oxy- 

 gen as a by-product. These interactions may be shown by the 

 following equations : 



