INDIAN TOBACCO 123 



occurs as epidermal outgrowths a short distance back of the 

 formative layer. The root-hair layer is progressive; as the 

 root increases in length new root-hairs are formed. 



Stems. The stems of dicotyledons may increase yearly 

 in diameter because they have open vascular bundles arranged 

 as a continuous ring of tissue surrounded by the cambium 

 cells', these divide to form new vascular tissue on the inside 

 and new cortical tissue on the outside. The cortex is of 

 variable thickness and when breaks occur or tissue is thrown 

 off because of the increase in diameter of the stem, the 

 phellogen cells divide to form parenchyma cells on the inside 

 and cork cells on the outside. 



Leaves. The leaves of dicotyledons usually have an intri- 

 cate system of venation, the larger veins connected by smaller 

 ones to form a network, resulting in a net-veined leaf. The 

 ends of the veins are usually free near the margins, forming 

 the open venation characteristic of dicotyledons. A great 

 variety of margins is possible in open- veined leaves. The 

 variations in margins are shown by Fig. 93, Part II. 



Flowers. The flowers have a cyclic arrangement, and they 

 are usually pentamerons or in fives, having five sepals, five 

 petals, ten stamens in two sets, and a five-carpeled pistil. 

 The greatest possible variation occurs, however, not only in 

 color, form and position of the floral circles but in the number 

 of units in each. Many of these variations being special 

 adaptations for bringing about pollination by insects. Refer 

 to Part II. 



