THE LEAVES OS 1 WEEDS. 33 



through the pith. Only a few of our weeds belonging to the grass, 

 sedge, lily and rush families have stems of this kind. In the other 

 class called exogens, or outside growers, the stem is composed of 

 distinct layers which surround one another in circles, and are 

 usually known as bark, wood and pith. All our Weeds except those 

 of the four families above mentioned belong to this class. 



The direction of growth of the stem is an important distinguish- 

 ing character of weeds. If, like the corn cockle, it stands upright 

 it is said to be erect; if arising obliquely from a prostrate base it 

 is called ascending. Stems which grow along the ground without 

 rooting are prostrate (purslane) or trailing (ground ivy). The 

 stems of some weeds, as the plantain and dandelion, are very short, 

 the leafless flower-stalk springing from the midst of a clump of 

 basal or so-called root-leaves. Such a flower-stalk is called a scape. . 



Some perennial weeds produce both ordinary erect and creep- 

 ing underground stetms, the latter being called rootstocks or 

 rhizomes. They may be known from true roots by their bearing 

 buds at short intervals. When the conditions are favorable these 

 buds produce erect above-ground stems. Such perennials are the 

 most difficult of all weeds to eradicate. The stems of some weeds, 

 as the cinquefoil, produce above ground and near the base runners 

 or offsets which take root and form new plants. 



The leaves op weeds. — Leaves are among the most important 

 parts of a weed, as it is in their cells that all the food of the plant 

 is assimilated or fitted for growth and for forming the flowers and 

 seeds. It is the leaf also which has the sole power of absorbing 

 carbonic acid gas from the air and by the aid of the sun's heat 

 and light, in the presence of the green coloring matter (chloro- 

 phyll), changing it into starch, this being used in forming other 

 tissues such as wood or seeds or tubers. In these tissues the energy 

 from the sun's heat and light is stored and when the tissues are 

 burned or used as food by animals the energy is set free and can be 

 controlled by man. Most of the energy used by man and animals 

 in performing the work of the world was at one time thus gathered 

 by leaves and stored in the roots or stems or fruits of plants. 

 Leaves are the principal organs of respiration or breathing which 

 the weed possesses. It is through them also that the excess of water 

 gathered by the roots passes off. Constituting as they thus do the 

 organs of digestion, breathing and transpiration or sweating, it is 

 very plain that.if one can prevent a weed from producing leaves it 

 will soon die. 



The broad expanded part of a leaf is called the Uade and the 



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