388 



TREES AND SHRUBS. 



378. The Poplars (Populus) and the Willows (Salix) 

 are closely allied, forming a family (Salicaceae) which is 

 sharply marked off from all other catkin-bearing plants. A 

 catkin is simply an inflorescence consisting of a main stem 

 or axis which bears either male flowers or female flowers, 

 each flower arising in the axil of a leaf (bract) and having 

 a very simple structure. The male and female catkins are 

 on separate plants. 



379. The best known Poplar is the Black Poplar, 



which we shall take as a type for first study. The main 

 part of the root system grows deep into the soil, but 

 some of the roots run along, more or less horizontally, 

 a little below the surface. The shallow roots often send 

 up leafy shoots (" suckers "). The tree is rarely over 

 a hundred feet high; most of the branches bend up- 

 wards, and the tree has a loose appearance and a " lopsided " 

 crown. 



The leaves (Fig. 156) have long stalks and the upper part 

 of the stalk flattened at right angles fco the blade, so that the 



leaf hangs loosely and quivers 

 in the gentlest wind. It has 

 been suggested that two ad- 

 vantages arise from the trem- 

 ulous character of the leaves 

 in this and several (not all) 

 other Poplars : (1) the resis- 

 tance offered to the wind is 

 lessened, so that the thin twigs 

 can better withstand the 

 strain ; (2) the evaporation of 

 water from the leaf is in- 

 creased, an advantage in 

 rapidly- growing trees with a 

 good water-supply from below. 

 The first suggestion is sup- 

 ported by the fact that in 

 Poplars with thick twigs, e.g. 

 Lombardy Poplar, the leaf -stalks are not flattened ; the second 

 by the fact that Poplars " prefer," i.e. thrive best in, deep 

 moist soils. 



Fig. 156. Twig and Male Catkin of a 

 Poplar. 



