248 HOST PEXETRATIOX 



entrance do so by the same means. Aronescu ( 1934) concluded 

 that both chemical action and mechanical pressure are necessary 

 for penetration bv the fungus causing black spot of roses. There 

 may exist only the two general means of penetration that have 

 been discussed, but perhaps each pathogen has made such modifi- 

 cations and adaptations as are suited to its own requirements. 



STOMATAL PEXETRATIOX 



Stomata constitute normal portals for entrance by a large num- 

 ber of pathogenic species. Observations on penetration through 

 stomata have been recorded for many different fungi. Such ob- 

 servations may be made by one of three methods: (a) epidermal 

 stripping, (b) sections of fixed, embedded material, and (c) use 

 of a stomatoscope. Certain advantages and disadvantages attend 

 the use of each. 



If spores are sown in drops of water on leaves and chalk is added 

 to indicate the site of the drops, it is not difficult to strip off epi- 

 dermis in the areas marked by deposits of chalk, mount it in water 

 with the exterior surface upward, and examine it under the micro- 

 scope. With a little practice the investigator can learn to tear the 

 leaf and thus strip off the epidermis, or to cut it off by holding 

 the leaf taut over the end of the finger and slicing parallel to the 

 leaf surface. Bv this method many examinations can be made in a 

 comparatively short time, and the time interval involved in pene- 

 tration can thus be determined. Mounting specimens in cotton 

 blue * instead of water may aid in differentiating the hyphae and 

 in clearing the host tissues. 



The merit of fixing, at known intervals after inoculation, tissues 

 which have had spores applied to their surfaces has the feature of 

 permanency to recommend its use. These tissues may be em- 

 bedded, sectioned, stained, and examined whenever time is avail- 

 able and may be kept indefinitely. This method, however, is 

 obviously both laborious and time-consuming. 



Direct examination with an apparatus known as the stomato- 

 scope requires familiarity with the operation of an apparatus that 

 has been available to only a few investigators. Pool and McKay 

 (1916) used such an appliance in their studies of penetration of 



'Use 0.1% cotton blue in lactophcnol, which contains equal parts of 

 phenol, lactic acid, glycerin, and distilled water. 



