2.41 THE STRUCTURE OF ECONOMIC PLANTS 



development by a mechanical breaking of the stem over a five- to 

 ten-day period, and that it also occurs in nature due to wind and 

 other mechanical factors. In small healthy plants, he found the 

 fibers free from bifurcation and other abnormal form changes. 



The Preparation of the Fiber. — In the preparation of the 

 fiber, the plants are harvested, shocked and dried before retting is 

 undertaken. This is accomplished by dew-retting or water-retting, 

 the former being more common in the United States and the latter 

 in Europe and Asia. In dew-retting, the stalks are spread out on 

 the ground and subjected to moisture or to freezing and thawing; 

 while in water-retting, they are placed directly in streams, ponds, 

 or tanks. During this period, bacteria and fungi act upon the 

 middle lamellae of the fibers and on the tissues surrounding them. 

 This is followed by breaking and skutching to remove the disinte- 

 grated tissues from the fiber, which is then hackled by hand. 



It should be noted that the technical hemp fiber of commerce is 

 not a single fiber cell, but consists of a group of cells which are 

 held together by their middle lamellae. One of the problems of 

 retting to obtain desirable commercial fiber is to insure the dissolu- 

 tion of the middle lamellae which connect the fiber strands with 

 adjacent tissue, and at the same time avoid a complete dissolution 

 of the middle lamellae between adjacent fiber cells. 



The Leaf. — The blade of the leaf is relatively thin and the 

 principal veins form prominent ridges on the abaxial surface while 

 the adaxial surface is depressed into a groove above each v^in. 

 (Fig. ii6.) The cells of the upper epidermis are considerably 

 larger than those of the lower and much more heavily cutinized. 

 Stomata are infrequent or lacking on the upper epidermis and very 

 numerous in the lower one. Briosi and Tognini (7) made several 

 stomatal counts for the cotyledons, foliage leaves, stipules, and 

 bracts, which are here summarized: 



TABLE III 



