SHOOTS. 



76 



62. The rootstock type. — This is prob- 

 ably the most common form of subter- 

 ranean stem. It is elongated, as are foli- 

 age stems, and hence the scale leaves 

 are well separated. It is prominently 

 used for food storage, and is also admirably 

 adapted for subterranean migration (see 

 Kg. 68). It can do for the plant, in the 

 way of migration, what prostrate foliage- 

 bearing stems do, and is in a more protected 

 position. Advancing beneath the ground, 

 it sends up a succession of branches 

 to the surface. It is a very efficient 

 method for the "spreading" of plants, 

 and is extensively used by grasses in cov- 

 ering areas and forming turf. The persist- 

 ent continuance of the worst weeds is often 

 due to this habit (see Figs. 69, 70). It 



is impossible 



Fig. 65. Branch buds 

 of elm. Three buds 

 (k) with their over- 

 lapping scales are 

 shown, each just 

 above the scar (&) 

 of an old leaf. — 

 After Behrenp. 



Fig. 66. A bulb, made up of overlap- 

 ping scales, which are fleshy on 

 account of food storage. — After 

 Gkat. 



to remove 



all of the 



indefinitely 



branchin g 



rootstocks 



from the soil, 



and an}' fragments that remain 



are able to send up fresh crops 



of aerial branches. 



5.3. Alternation of rest and 

 activity. — In all of the three 

 stem types just mentioned, it 

 is important to note that they 

 are associated with a remark- 

 able alternation between rest 

 and vigorous activity. From 

 the branch buds the new leaves 



