366 TRANS. ST. LOUIS ACAD. SCIENCE. 



either a few stomata, often difficult to discover, or, rarely, numer- 

 ous ones ; so that in this interesting species the question arises 

 whether the presence or absence of stomata alone can specifically 

 separate forms otherwise scarcely distinguishable, as that acute 

 • observer of these plants, A. Braun, has maintained, or whether 

 the stomata do not always play that important part in classifica- 

 tion generally assigned to them. Below will be found directions 

 for the investigation of the stomata. 



The parenchyma of the leaf consists of a few or several layers 

 of chlorophyll-bearing cells, i. under the epidermis, 2. around 

 the central bundle of vessels, and 3. forming the dissepiments, 

 which cross each other in the centre of the leaf. 



An important element in the leaf structure is found in the peri- 

 pheral bast-bundles, which are present in some and absent in 

 other species ; and their presence often, but not always, coincides 

 with the presence of stomata. When present they commonly 

 form four bundles^ two in the two anterior angles of the leaf, 

 and two where the median dissepiments connect with the ante- 

 rior and the posterior wall of the leaf; in /. Nuttallii I find 

 only three bundles, the anterior median one being wanting ; in 

 /. Cubana six bundles are visible, the two additional ones being 

 located where the transverse dissepiment unites with the outer 

 wall. In some rigid-leaved land species, e.g. /. melanopoda, 

 often several smaller accessory bundles are found scattered under 

 the epidermis. 



The examination of the fresh Isoetes leaf is not very difficult ; 

 particles of the epidermis are easily removed and show the 

 stomata, when present, very distinctly. Where there are few 

 stomata, the epidermis from different parts of the leaf must be 

 examined, and especially from the tip, as they are more apt to 

 be found there. In dried specimens the leaf must be soaked, the 

 alg(^ which often adhere to the surface have to be carefully 

 scraped off, after which I make several sections \ ox \ line wide, 

 lay them open by a vertical slit, detach the central bundle, and 

 then scrape very gently the inner surface so as to remove the par- 

 enchymatous cells which obscure the appearance of the stomata 

 This process can be aided by an immersion of the specimen in a 

 weak solution of caustic potash. The work is often a difficult 

 •one when the specimen is very old or poorly preserved, and 



