ir>, 

 from tlieir imsj^al cells and .iius loave tlio leaf nia^J:ecl#Tlie 

 later development and mature structui-e of theso tricliomes 

 have been studied by Russov/, tind have been worked ou' in 

 fl-reat detail by Meunier for the similar ones of Pilularia. 



Stomata also occur on tlic petiole, but apparently 

 not till quite a late stago and their development v/as not 

 studied. 



By th.e time that the longitudinal partitions are three 

 or four cells in radial v/idoh, a longitudinal rov/ of the 

 large mesophyll cells opposite each partition, usually in 

 the next to the inner layer of these, becones specialized 

 to for;i one of the so-called tannin sa.cs (t.s.^Figs. IS, 

 19). Tlie contents of tiiese are distinguished by a brovmisli 

 color in the ujistained alcoholic material, but usually 

 stain very darkly v;ith haemalnm. In the mature leaf their 

 cells are about the same size as the suTroujnding mesopliyll 

 cells. TliG latter are cibout tv/ice as long as broad and 

 fit together quite squarely at the ends, 'vhile laterally 

 they are rovinded off from each other, leaving betv/een tlicm 

 many small intercellular spaces (Fig. 20) v/hich coiinect 

 with the large air canals. 



IVliile tliesG structures have been developing from the 

 dermatogen and periblem layers of the petiole, ilie plerome 

 portions of tjii first four sections have given rise to the 

 axial vascular bimdle. Tlie first plerome cell cut off from 



