94 



The followiiifi sini])le inodiflcation of this ohxssic experhiient will he 

 instructive and will usually set the student to tliinking to some purpose. 

 I're])are several round disks from oi)a(|ue ])aper or li^lit cardboard. These 

 may he readily made of ahout the riuht size witli a Kunwad cutter. Se- 

 lectini; a broad leaved plant of suitable size, place a pair of the disks one 

 on each side of the leaf opposite each other and in close cont^irt witli the 

 surface of the leaf, and fastened in jilacc liy a li'^dit s])rin.i,' cliii. such as is 

 connnonly used to fasten loose sheets of pai)er. Attach anotlier i)air of 

 disks in a sinnlar manner but supported from the surface of the leaf by 

 wax feet or other sup])orts so as to allow a free circulation (if air lictwecn 

 the di.sks iind the leaf. Now select a couple of cover t^lasses, such as are 

 used in microscoi)ic work, of a]iproximately the same size as the paper 

 disks and attach these in a similar manner so as to be m close conflict 

 with the surface of the leaf. The plant is now exiiosed to the suuli,u;ht and 

 after a time the leaf is tested for starch in the usual way witli iodine. 

 The result as oi»tained recently by my own students is shown in the ti.mu'e. 



As is seen the place of the ojiaque disks is marked by a spot free from 

 starch and bounded by a sharply defined line approximately at the edire 

 of the disk in both cases, showing of how little significance in this form of 

 the experiment is the question whether the stomata are covered or not. 

 On the other hand, the glass disks are marked ))y only a small s\wi at the 

 center without starch and the intensity of the starch reaction shades off 

 gradually from the margin toward this cenlial spot so that the boiuulary 

 of the disk is not evident. It is hardly retcssary to add that some care 

 sliMuld hi- exercised in timing the ex])osure to i1h> suidight in order to get 

 the most striking results. The phenomena afford a neat little problem for 

 the student to explain. With a little thought most students can do this if 

 they understand clearly th(> srructure of tlie leaf. 



The experiment may be furthi'i' elaborated with instructive results by 

 selecting leaves with stomata (lelinitely distributed as upon only one side 

 of the leaf and by using the disks, both the glass and oi>aque. singly 

 instead of in pairs. 



Dc/'auiv UnircrxUi/. 

 GreencaxtU-, ind. 



