270 HISTOLOGY OF STEM OF THE WAX PLANT 



3 jj mm. The ends of the fibers are generally prolonged into slender 

 threads, but in some cases the termination is quite abrupt, and they 

 expand slightly near the end as shown on pi. II, fig. 8b. Sulphuric 

 acid causes the fibers to swell, and contract violently in the direction 

 of their length, and finally dissolves them. Schultze's solution changes 

 them to a deep azure-blue color ; while iodine shows a slight yellow 

 tinge. They contain a fluid that stains readily with magenta and 

 iodine, and does not appear to be affected by sulphuric acid. Scat- 

 tered here and there in this band of parenchyma will be seen crystals, 

 sclerotic cells, and milk cells (latex cells). These are in all respects 

 similar to those found in the outer band, and a detailed description 

 of them need not be repeated. 



Cambium 



The cambium cells are composed of exceeding delicate cellulose 

 walls and by successive division form the wood, medullary rays, etc. , 

 on one side, and the tissues that surround them, on the other. The 

 development of these tissues has not been attempted. The cells 

 are generally about 15 ^ in cross section and 50 n in length. 



Wood 



Fritz Miiller, in an interesting paper in the Botanische Zeitung, 

 February, 1866, describes the structure of the wood in climbing 

 plants about Disterro and gives figures showing the irregular form that 

 it sometimes assumes. He states that these irregularities are due to 

 the separation of the woody portions or to "strings of soft tissue 

 drawn through them," the stem becoming thereby more flexible 

 than if the mass of wood was regularly formed. 



In the young stem of Hoya carnosa the outer boundary of the wood 

 is quite regular and passes in to the cambium in the ordinary manner. 

 But as the stem increases in age, the growth is more and more irregular, 

 till in a stem about 10 mm. in diameter the appearance of a section is 

 like that shown on pi. II, fig. 1, 4; the wood being generally much 

 thicker on one side than on the other and penetrated in many places 

 by wedge-shaped masses of parenchyma. I have not been able to 

 decide whether the wood separates and allows the soft tissue to pene- 

 trate it, or whether it simply grows irregularly around them, but I 

 think the former, as in the young portions of the stem the wood makes 



