72 PHOTOSYNTHESIS AND RESPIRATION. 



darkened seedling. Some further experiments will serve to 

 emphasise this point. 



* Remove (a) one cotyledon, (b) both cotyledons, from soaked Broad 

 Bean or Scarlet Runner seeds just beginning to germinate. Place some 

 of these seeds, along with untouched seeds, in the light ; place others, 

 also with untouched seeds for comparison, in darkness. 



* Remove the foliage- leaves from (1) a young Bean plant which has not 

 yet used up the food in its cotyledons, (2) an older plant whose cotyle- 

 dons have fallen off (if they have shrunk considerably, but are still 

 attached, pull them off). Does the removal of the foliage-leaves check 

 the growth of the plant, as compared with that of similar plants left 

 untouched? In which case, (1) or (2), is the effect greatest? 



106. Comparison of Cotyledons and Foliage-Leaves. 



We have seen that the cotyledons of the Broad Bean 

 resemble the foliage- leaves in the fact that branches arise in 

 their axils. By growing a seedling in coloured water it can 

 be shown that the cotyledons have veins, which can also 

 be seen in sections examined with the microscope. It is 

 always a useful plan to tabulate i.e. to write down in 

 parallel columns the chief differences between different 

 plants or different parts of the same plant, as regards their 

 position, form, structure, etc. and then to find out as com- 

 pletely as possible, by observation and experiment, the 

 reasons for these differences. In comparisons of this kind 

 always try to discover whether differences in structure can be 

 explained as due to differences in function. 



Cotyledons. Foliage-Leaves. 



opposite alternate 



small large 



simple compound 



pale yellow green 



thick thin 



veins obscure veins conspicuous 



no stipules stipules present 



remain below ground formed above ground 



gradually turn smaller gradually turn larger 



fall off after a time remain on a long time 



Can you add any further differences ? Try, as you proceed 

 with your study of the green leaf and its work, to explain 

 these differences in form, texture, duration, etc., between 

 the cotyledons and the foliage-leaves of the Broad Bean. 



