598 BOTANY OF CROP PLANTS 



same type. In the Sumatra cigar-wrapper tobacco the 

 leaves range from i6 to 30, in the White Burley plug tobacco 

 from 10 to 18, and in the Zimmer Spanish cigar-filler tobacco 

 from 14 to 20. 



Tobacco plants are sometimes grown in the shade of tents, 

 which condition makes a larger and thinner leaf with less 

 vascular tissue. The leaf is thus improved for wrapper 

 purposes. The chief effect of shade is to reduce the rate of 

 transpiration. There is evidence that transpiration rate is 

 the important factor determining the development of vas- 

 cular tissue. 



^' Grain" in Tobacco Leaves. — "Grain" of tobacco appears 

 as small pimple-like projections or papillse over the cured 

 leaf. The papillae vary in size from about i millimeter to 

 microscopic dimensions. Each grain body consists of from 

 one to several leaf cells filled with crystaUine substance. 

 The grain is composed chiefly of calcium, with some potas- 

 sium and magnesium, in combination with citric and malic 

 acids. Grain of tobacco is developed during the process of 

 curing and fermentation. It is a character that the buyer 

 takes into consideration when he selects tobacco. 



Inflorescence and Flowers.— The inflorescence is a terminal 

 panicle. The flowers (Fig. 244) are about 2 inches long, and 

 pink, yellow, purple or white. The tubular or bell-shaped 

 calyx is four- to five-cleft. The tube of the corolla is swollen, 

 its lobes spreading and pointed. 



Pollination and Fertilization.^ Tobacco flowers bear nec- 

 taries and are visited by insects. Hence, cross-pollination 

 is probably somewhat frequent. Moreover, observations 

 and experiments show that the flowers are self-fertile — that 

 they will produce viable seed when close-fertilized. The 

 earlier blossoms of an inflorescence are more commonly 

 close-fertilized than are the later ones of the same inflores- 



