Vegetative Organs of Vascular Plants 137 



complete or even incorrect picture of the woody stem in the mind 

 of the student. 



Twigs are not satisfactory for making longitudinal sections in 

 quantities. Unembedded twigs cannot be held in the microtome 

 horizontally for longitudinal sections. If an embedded and blocked 

 twig is sectioned longitudinally, only the outermost sections are 

 strictly tangential, and only a few slices from the center are true 

 radial sections, cut parallel to a ray. For first-class preparations cut 

 accurately on the three desired planes, use blocks of wood and 

 attached bark removed from living trees as illustrated in Fig. 2.2. 

 Sectioning of such blocks is quite impossible without embedding in 

 celloidin; whereas, with the celloidin method, perfect sections can be 

 produced in quantities (Fig. 13.9 6, c). Collect the material in the 

 winter when the cambium is firm. Soft wood like basswood, white 

 pine, apple, or silver maple can be cut without special softening. Kill 

 in FAA, and embed in celloidin. Hard woods like oak or locust must 

 be treated with hydrofluoric acid after killing and hardening in FAA. 

 The protoplasts cannot be expected to be in perfect condition after 

 treatment in HF. The process is described in Chap. 8. 



Fig. 13.9— Illustrations of material cut l)y the celloidin method: a, cross section of 

 apple graft union; b, sapwood region of sector from 20-year-old trunk of Tilia, 

 sections in three planes; c, sections from approximately 40-year-old trunk of apple 

 tree. All subjects killed in FAA. The Tilia stem was infiltrated in Cellosolve solution 



of celloidin. 



