44 Practical Plant Biology. 



less soluble than carbon dioxide, it appears in the form of bubbles 

 at the surface of the cell. 



The equation above represents the initial and final stages of 

 the process, but it is highly improbable that there are no inter- 

 mediate steps in the formation of such a complicated carbohydrate 

 as starch. Various steps have been suggested with more or less 

 of plausibility attached to them, and more or less supported by 

 collateral evidence. Thus it has been supposed that formaldehyde 

 (CH 2 O) is one of those steps and that in its formation from the 

 carbonic acid formed by the union of water and carbon. dioxide, 

 hydrogen peroxide (H 2 O 2 ) is produced according to the equa- 

 tion 



H 2 CO 3 + 2 H 2 O = CHoO + 2 H 2 O 2 . 



Experience shows that these two substances formaldehyde and 

 hydrogen peroxide are highly injurious to plants. Hence we 

 must assume that they are removed before any sensible accumula- 

 tion of them occurs in the chloroplast. This removal is assumed 

 to take place by the almost instantaneous polymerisation or con- 

 densation of the formaldehyde to form a more complex carbo- 

 hydrate such as a hexose (C 6 H l2 O ti ). The protoplasm itself has 

 been held responsible for this change. The hydrogen peroxide, 

 on the other hand, is thought to be broken up by an enzyme 

 called catalase as quickly as it arises 



2H 2 O 2 = 2H 2 O + O 2 . 



The very general occurrence of this enzyme in plant cells may be 

 used as an argument that hydrogen peroxide is often produced. 

 It may be seen that we could not expect to be able to demonstrate 

 the presence of either the aldehyde or the peroxide by chemical 

 tests, but it has been found that cells with chloroplasts are able 

 to build up starch from aldehyde supplied in minimal doses. 1 



1 It has been thought since the cane sugar (sucrose C 12 H 02 O n ) content of 

 some green cells rises immediately on exposure to light, while the hexoses 

 (C B H 12 O 6 ) do not increase, that sucrose is a preliminary step to the formation 

 of hexoses, and that by the agency of invertase the sucrose is transformed to 

 these sugars. But it is not necessary to accept the improbable suggestion 

 that the more complex sugar is the first formed. Supposing the hexose to 

 arise in a very small cavity of the protoplasmic gel, or to be adsorbed upon 

 the surface of its colloidal particles, even though in very minute quantities, its 

 concentration might be great, and hence in presence of the enzyme a small 

 amount of sucrose will b- formed. If a mechanism exists in the cell for 

 transporting this sucrose from the point of manufacture to a cavity or vacuole 

 in the protoplasm isolated from the sphere of activity of the invertase, it is 

 evident that a continual rise of the total amount of sucrose in the cell will be 

 observed, while the amount of hexoses in it may remain very small and ap- 

 proximately invariable. 



