HISTOLOGY OF STEM OF THE WAX PLANT 275 



Pith 



The pith is composed of spherical cells from lo to 50 ix in diameter, 

 being generally filled with starch and having thin cellulose walls 

 without markings, as far as I have been able to observe. In the center 

 are a number of sclerotic cells similar to those found in other parts 

 of the plant, but more regular in shape. These form the axis of the 

 plant, growing upward in advance of the sclerotic girdle. They are 

 of the same size as the cells of the pith and are developed like those 

 of the girdle by the thickening of the cell wall. Crystals also occur 

 in abundance, and are sometimes surrounded by the sclerotic cells. 

 They have a nucleus of organic matter and often completely fill the 

 cell that holds them. Around the pith will be noticed clusters of 

 small cells presenting the appearance shown on pi. II, figs, i, 4. 

 These, I think, are what Davids calls Weichhasthundels. They 

 are about 10 jjl in diameter and 50 ^i in length. These cells are filled 

 with a colorless fluid and bounded by thin cellulose walls. They 

 are sometimes arranged around the pith, forming four well-marked 

 patches, as shown in the figure referred to. 



Such is a general account of the structure of Hoya carnosa. If we 

 glance for a moment at its methods of growth, we at first see the young 

 stem with the column of sclerotic cells in the center, which, as it 

 advances, is followed by the girdle which envelops the tissues within 

 it and furnishes a rigid support to the plant with the least expense of 

 material, and, if my supposition prove correct, disappearing when the 

 wood has reached a sufficient thickness to render its presence super- 

 fluous. 



I wish in conclusion to acknowledge my obligations to Dr. N. L. 

 Britton and others who have kindly assisted me with advice and sug- 

 gestions. 



EXPLANATION OF PLATES 



Plate I 



Fig. I. Cross-section of young stem showing the different zones of tissues men- 

 tioned on page 4. 



Fig. 2. Hair from young stem. Page 7. 



Fig. 3a. Transverse section of a stoma. Fig. 4. Diagrammatic showing develop- 

 ment of cork, p = phellogen. c = cork. Page 8. 



Fig. 5. Epidermis of stem as left after action of sulphuric acid. 



