136 



ROOTS AND UNDERGROUND STEMS 



onion ; 281, vertical 

 section of a tulip bulb, 



become dry and papery, and observe how the inner fleshy 

 ones encircle one another successively. 

 A bulb of this kind, made up of succes- 

 sive layers, is said to be tunicated. Look 

 for the flower cluster in the center. Do 

 you find any axillary bulbs ? Any axil- 

 lary scapes? Compare the concentric 

 rings of a tunicated bulb as seen in 

 cross section with those of the beet; 

 are they of the same nature ? Before 

 answering, look again at your cross sec- 

 tion of the lily bulb and think what 

 would happen if the scales were to be 

 broadened sufficiently at the base for 

 each one to encircle completely all within 



bulbs' ^GRAY) 1 ? 10 ^, it. Compare the leaves and scales of 



cross section of an t j le O mon With the leaf- 



stalk of the sycamore 

 (Fig. 20), and see if you 

 can find any reason for 

 regarding them as modified petioles. 



195 . Uses of Underground Stems. Though 

 the chief function of underground stems is 

 the storage of nourishment, they serve other 

 purposes also. In plants like the ferns, that 

 require a great deal of moisture, and in 

 others growing in dry places, like the blackberry lily, that 

 need to husband it carefully, they may be useful in pre- 

 venting the too rapid evaporation that would take place 

 through aerial stems. Defense against frost, cold, heat, 

 and other dangers, as well as quickness of propagation, 

 are also attained, or assisted by this means. 



PRACTICAL QUESTIONS 



1. Name some plants in your neighborhood that are propagated by 

 rootstocks ; by rhizomas ; by tubers. 



2. What is the advantage of propagating in this way over planting 

 the seed? (192, 195.) 



282. Leaf of 

 an onion divided 

 lengthwise. 



