214 EXPERIMJ^NTS WITH PLANTS 



We have already learned how a coat of vaseline 

 stops evaporation from the leaf. Do you find leaves 

 coated with water-proof substances, such as resins, gums, 

 varnishes, wax, etc.? Notice especially the varnishes, 

 gums, etc., on buds, which prevent them from drying 

 up. Remove the scales, which are covered with var- 

 nish, and note the result. Many leaves have a coat of 

 varnish during their youthful period but dispense with 

 it as they grow older and less sensitive, and the epi- 

 dermis thickens (Cherry, Poplar, Birch, Alder, Peach, 

 etc.). 



Take two leaves of the same size which are thickly 

 coated with wax (the young horizontal leaves of Euca- 

 lyptus are especially good) ; remove the wax fron one 

 by rubbing with a soft cloth ; seal the cut surfaces with 

 vaseline, place a leaf in each pan of the balance, and 

 balance them by adding weights to the proper pan. 

 Observe from time to time which is losing water more 

 rapidly, as indicated by the rising of the pan. 



It will be noticed that leaves (Iris, Bamboo, Dusty 

 Miller, Oxalis, Silver-leaved Poplar, Blackberry, etc.) 

 which have a covering of hairs, wax, etc., have a glis- 

 tening silvery appearance under water ; this is due to a 

 layer of air which is held tenaciously. On dipping such 

 leaves in water we find that the stomatic surface is so 

 protected that it does not become wet; this is an advan- 

 tage, since, if water penetrated into the stomata, it 

 would completely prevent the passage of air and the 



