680 MINNESOTA BOTANICAL STUDIES. 



difficulty. No permanent mounts were made. To insure ac- 

 curacy, most of the drawings were made with the aid of a cam- 

 era lucida. 



List of plants investigated. — The following plants were ex- 

 amined: Anogra pallida (Lindl. ) Britton, Galpinsia hart- 

 wegii (Benth.) Britton, Kneiffia fruticosa (Linn.) Raimann, 

 Kneiffia glauca (Michx.) Spach, Kneiffia linearis (Michx.) 

 Spach, Kneiffia linifolia (Nutt.) Spach, Kneiffia pumila (Linn.) 

 Spach, Megapterium missouriensis (Sims) Spach, Meriolix 

 serrulata (Nutt.) Walp., Oenothera humifusa Nutt., Oeno- 

 thera sinuata Linn., Onagra biennis (Linn.) Scop. 



Epidermis. — The cells of the epidermis are similar in all the 

 species examined. In cross section they are square or more 

 often oblong, the walls are rather thick, the outer wall is 

 bulged. In surface view the cells appear more or less oblong, 

 narrowed at the ends, or they may be described as elongated 

 hexagonal in outline. 



The dimensions of the epidermal cells are by no means con- 

 stant even throughout the same plant. Perhaps an average 

 cell is lOOmik. in length, with its tangential and radial diameters 

 25mik. and 20mik. respectively. As a rule no definite layer of cu- 

 ticle is to be recognized, nor is there auy considerable thicken- 

 ing of the outer walls of the cells. In some cases, however, 

 the outer wall is somewhat noticeably thickened and occasion- 

 ally shows, on a properly stained section, a distinctively strat- 

 ified structure (Kneiffia glauca). The chief differences notice- 

 able in the epidermis are in the length of the cells, the num- 

 ber, shape and size of trichome structures and the number of 

 stomata. This last character is, however, of no considerable 

 taxonomic importance since the number of stomata varies with 

 the particular plant and in any given plant with the part ex- 

 amined. 



The epidermis and the cortical layer are absent in older parts 

 of the stem, being pushed out by the developing cork. In cer- 

 tain species the cork begins growth very early; notable for 

 this peculiarity are Anogra pallida, Kneiffia fruticosa and lini- 

 folia and Galpinsia hartwegii. The first named is particularly 

 remarkable in this respect, and it is only in the very youngest 

 parts that the epidermis remains. 



The epidermal hairs are never branched. As a rule they are 

 straight or slightly curved, pointed at the end and commonly 

 unicellular. The hairs vary considerably in length, averaging, 



