Cannon : Anatomy of Phoradendron villosum Nutt. 375 





breadth. The youngest leaves which may be seen in early spring 

 measure .3 by .75 cm., while the next older ones are 1-1.25 by 

 1*75— 2 -2$ cm. There is a like difference in measurement between 

 the second and third sets of leaves, and also, the differences in area 

 of the first two seasons' leaves are generally paralleled by a differ- 

 ence in thickness. The leaves do not increase in thickness after the 

 second season. 



The tips of the youngest leaves are projected beyond the 

 growing point of the stem, and their dorsal surfaces are closely 

 pressed together. The surfaces thus in contact are inaccessible to 

 the light and are colorless, while exposed surfaces take on the color 

 characteristic of the plant. A cross section of such a leaf shows that 

 it has a well-defined epidermis, and a mesophyll composed of simi- 

 lar cubical cells with small intercellular spaces. The cuticle of the 

 dorsal surface is noticeably thinner than that of the ventral (Fig. 3) 

 and this, like the coloration, may be in some manner associated 

 with the length of exposure to the light. A relatively large num- 

 ber of epidermal cells on both sides of the leaves are prolonged 

 into long trichomes with bulbous bases, and both trichomes and 

 epidermal cells are well filled with finely granular protoplasm and 

 and are provided with large nuclei. The cells of the mesophyll, 

 in the second season, become elongated and palisade-like, and it is to 

 the lengthening of the mesophyll cells in a direction at right angles 

 to the surface of the leaf, that the increase in thickness, spoken of 

 above, is due. (Fig. 4.) In this regard P. villosum recalls Viscum 

 album. * The increase in thickness from the first to the second 

 season is as the ratio 1 : 2. f The measurements are .4 mm. for 

 the first, and .79 mm. for the following season. As the leaves 

 mature the cell walls become pitted, a large number of them are 

 found with crystal aggregates (calcium oxalate), and prominent 

 intercellular spaces are developed. Another characteristic of the 

 old leaf is the kind and position of the stomata. The stomata, 

 which are found on both sides of the leaves, are of the xerophytic 

 type described and figured by Schimper, t and while they are con- 



* Marktanner-Turneretscher, Sitzungs. d. k. Akad. d. Wissen. Math.-Natur 

 Classe, 91 : 5 Heft, May, 1885. 



t Warming, Pflanzengeographie, 302. 

 t Schimper, Pflanzengeographie, 7. 



