June, 1907.] Ohio Plants' with Pwidate Glands. 179 



have practically the same structure as the light glands, but are 

 made dark by the secretion which is contained in them. The 

 age of the leaf may also have some influence on the darkness of 

 the spot. 



The glands of Boebera papposa (Fig. 4) appear to the naked 

 eye as large yellow spots, oval in shape. The gland wall is very 

 thick being made up of a number of layers of cells like the 

 prickly ash. The cells composing the wall stain quite dark on 

 the outside, but the inner layers take the stain much less prom- 

 inently. The cells in the innermost layer are irregular in shape 

 and remain very light in color. They are sharply limited from 

 the adjoining layer by a definite heavy wall. The gland is a 

 little less than twice as long as broad, and extends from the upper 

 epideraial layer to the lower epidermis. 



The internal glands of this type usually secret volatile oils, 

 but the glandular peltate scales found on the surface of the 

 leaves and stems very commonly secret resin, although no ab- 

 solute distinction can be made. Gaylussacia resinosa (Fig. 8) 

 is provided with external scales on the under side of the leaf. 

 These scales are attached to the leaf by stalks which are made up 

 of about seven cells. The scale itself is divided into from six to 

 ten cells. The resin is secreted from this and as the scale becomes 

 mature the outer covering is pushed off by the secretion. This 

 produces a bulging out of the scale, and the whole structure is 

 surrounded by a mass of bright yellow resin. The illustration 

 shown in most texts of this type is the gland found on the bracts 

 of the common hop Humulus lupulus, but the hop gland is more 

 simple in structure. The cross-section of the leaf of Gaylus- 

 sacia shows no further adaptation for secretion, and even the 

 lower epidei-mal covering is perefctly normal. The longitudinal 

 section shown (Fig. 9) is cut through the base of the disc and 

 therefore shows one central cell of the stalk and eight surrounding 

 cells of the scale proper. These cells show rather large dark 

 nuclei and contain abundant cytoplasm. 



In Glecoma hederacea, common ground ivy, (Fig. 7), resin 

 is also secreted, but the position of the scale is somewhat differ- 

 ent. Instead of secreting scales on the surface of the epidermis, 

 the organs for this purpose are in depressions or pits. The 

 gland has a stalk of two cells and below a scale showing a row 

 of from three to six cells in cross-section. These cells stain quite 

 dark but the nuclei can be seen without difficulty. The outer 

 wall of the scale is separated from the protoplasm of the cells 

 by a definitely limited clear space. 



Besides the glandular tissue on the inside and glandular 

 scales, other glandular structures are developed from the epi- 

 dermis which represent true epidermal hairs or emergences. 

 The hairs may be of different types, unicellular or multicellular. 



