BULLETIN OF THE BUSSEY INSTITUTION. . ook 
are a good many air spaces, some of which connect with the external 
air by means of the breathing-pores. There are also in certain parts 
of the leaf bundles of vessels or very long cells with thickened walls 
marked with rings, spirals, &c.; but, in the present article, we need not 
consider them. 
Fig.L; 
Suppose, now, we make a section through one of the brown spots 
on the leaf of a plant affected by the rot. We notice that the cells 
are of about the same shape as in the healthy leaf, but the chlorophyl- 
grains have lost their bright green color and are, in some places, evi- 
dently becoming disintegrated. We notice, also, something which was 
not seen at all in the healthy leaf. A number of branching threads, 
represented in Fig. 1, a, a, are seen running between the proper cells 
VOL. I. 41 
