358 



PHYSIOLOGY OF NUTRITION. 



selves acquainted somewhat more accurately with the structure of 

 sieve tubes, especially with that of their sieve plates. 



We prepare transverse sections from a stem of Cucurbita Pepo 

 (alcohol material) 10 mm. in thickness. Under a low power we 

 observe the epidermis, the interrupted collenchyma, the cortex, 

 the ring of sclerenchyma, and the vascular bundles grouped in a 

 double circle. These are made up of a wood portion with very 

 wide vessels, and an inner as well as an outer bast portion. The 

 inner bast or sieve region clasps the inside of the wood or vascular 

 region in a crescent-like manner. For more accurate examination 



FIG. 117. Cucurbita Pepo. Portions of sieve tubes from alcohol material. A, in 

 transverse section ; B-D, in longitudinal section. A, a sieve plate seen from above ; B and C, 

 two elements seen from the side ; D, the connected contents of two elements after treat- 

 ment with Sulphuric acid; s, companion cells; it, strand of mucilage; pr, protoplasmic 

 tube; c, plate of callus; c*, small plate of callus on one side of a lateral sieve area, Magn. 

 540. (After Strasburger.) 



under higher magnification, it is well, according to Strasburger, 

 to lay the sections in aniline blue for a short time, and then 

 mount them in a drop of glycerine. The tissue of the inner and 

 outer bast is made up of wide sieve tubes, of their companion cells 

 with contents stained blue (see Fig. 117), and finally of cambi- 

 form cells. The perforated sieve plates are easily recognised 

 where the sections have passed through them. 2 



