BOTANY "i THE LIVING PLANT 



, K . „ ■• ,,„ pai the leaf, starch formation 



wiI! to those parts. To a demonstration of 



thi il may be prepai rom a piece ol opaque material, such 



,1 t0 the upper surface of a starch-free leaf. After 



• J hoi I the leaf to light, application of the iodine 



. thai only the areas that were exposed to light will 



blue-black colour, indicating that photosynthesis has 



limit, parts (Fi( 78). Clearly the effect of light is 



light r . 1 be used in photosynthesis only by those 



that tually illuminated. 



md plant depends on the atmosphere for the carbon dioxide 



i n p h, This is indicated by an experiment in which 



r. previously freed of carbon dioxide, is passed through 



a bell-jar which 1- d to a glass plate and contains a number 



with their stalks in water. The leaves should be initially 



;.. 117). Although the bell-jar is placed in a good 



light, a subsequent test will show that under these conditions no 



photosynthesis occurs. Alternatively, normal air may be passed 



illuminated leaves and the issuing air shown to contain less 



1 dioxide than it did originally. 



rbon dioxide is present in the atmosphere to the extent of 3 volumes 



r 10.000 volumes of air. This is an average figure and may be departed 



from at certain times and places. Thus in the neighbourhood of extensive 



n the carbon dioxide-content of the atmosphere may during the 



middle of the day be appreciably below the figure mentioned, owing to photo- 



' tin. there is a constant evolution of carbon dioxide from the 



.1. due to the respiration of soil organisms, and the proportion of the gas 



present in the atmosphere Dear the soil is liable to be considerably above 



the aver are. The average figure is kept constant because the various 



proce ting it, photosynthesis and plant respiration being the chief, 



equilibrium. The amount of carbon dioxioc available to the plant 



it tirst sight to be very small, though it must be remembered that 



if mo I the ^ absorbed at any point their places are taken by 



others diffusing from neighbouring regions of the atmosphere. It is, how- 



:i that tin- photosynthetic process is often hindered by the low 



propo rti on of carbon dioxide that is present in the atmosphere (see p. 119). 



In order to reach the actual site of photosynthesis, the molecules 



of carbon dioxide musl first diffuse through the stomatal pores 



Fig. 79)i these being usually open in daytime, to which period 



naturally restricted by considerations of lighting. 



or vapours can be absorbed or evolved by the leaf tissues 



through the stomata with surprising rapidity (see p. 104), and it is 



