Vegefafive Organs of Vascular Plants 135 



and furnish long hypocotyls as well as epicotyl materials. Do not use 

 FAA; kill in Craf II, and use alcohol-xylene for tender stems and 

 butyl alcohol for tough ones. Tomato and tobacco seedlings grow 

 slowly, but they are almost indispensable subjects. Potato plants are 

 easily grown from tubers. Stems of these plants are not killed properly 

 by FAA but are preserved with excellent cellular detail in Craf II. 

 Old, tough stems of tomato and tobacco must be processed in TBA or 

 sectioned in celloidin. Small potato tubers, 3 to 6 mm. in diameter 

 are easy to section. Kill in Craf I, and embed in paraffin by a slow, 

 closely graded process. Longitudinal sections show that the tuber is 

 a stem with an apical meristem which produces leaf primordia. 



Medullary bundles and anomalous cambial activity occur in the 

 Chenopodiaceae. The common weed Chenopodium album is probably 

 the most readily available representative. Several related weeds are 

 equally interesting. Kill in FAA or Craf III, and process in butyl 

 alcohol or dioxan. 



The foregoing methods recommended for specific herbaceous 

 stems can be used with an extensive range of plants in many species of 

 economic importance or academic interest. For instance, commercial 

 fibers of primary and secondary derivation can be illustrated with the 

 stem of Cannabis saliva (Fig. 13.8). As a broad general recommen- 

 dation, use a mild chrome-acetic-formalin on tender materials, and 

 process in alcohol-xylene or acetone-xylene. For moderately hard 

 stems use Craf III, and for very hard stems use FAA, followed by 

 dehydration and infiltration in dioxan or butyl alcohol. 



The bush fruits like raspberry, blackberry, currant, gooseberry, 

 and other plants having similar semiwoody stems may be handled like 

 herbaceous stems while in the tender growing stages, but they 

 eventually become too hard to process by the foregoing methods. Such 

 hard materials usually must be handled like woody stems, as 

 described in the following pages. 



For the study of twigs of woody plants, material collected during 

 the winter has some advantages. The previous season's xylem is fully 

 lignified, secondary phloem is fully matured and firm, the cambium 

 is clearly distinguishable as a layer immediately adjacent to the wood, 

 and the cambium does not slip readily. However, if the development 

 of cambial derivatives is to be studied, stems must be collected at 

 intervals during the growing season. Such materials must be processed 

 with greater care than dormant stems. Twigs should be taken to the 

 laboratory promptly and cut into short pieces for killing as described 

 in Chap. 2. 



