The Green Plant 497 



When particles of a fine yellow dust, called pollen, are carried from 

 one flower to another, or from one part of a flower to another 

 part, by means of the wind or by certain insects, they cause the 

 egg cells of the flower receiving the pollen to grow into fertile 

 seeds. These seeds are then released by the plant, and under 

 favorable conditions will develop and become mature plants. 



Leaves. Leaves are the chief agents by means of which the 

 plant relates itself to light and air. A typical green leaf con- 

 sists of three parts, the blade, the petiole, or leaf stalk, and a pair 

 of leaf-like organs called stipules situated at the base of the leaf 

 stalk. The blade is the broad, flat part of the leaf, the part best 

 adapted to receive the largest amount of light. 



If you observe leaves carefully you will see that they have a 

 framework composed of fibers that penetrate the whole inner 

 surface and provide a place of attachment for the more delicate 

 parts. These fibers, or ribs, are called veins. They are the chan- 

 nels through which fluid circulates to every part of the leaf. In 

 some leaves there is one larger fiber or vein called the midrib 

 which separates each leaf into two similar halves. 



Although these veins are arranged in many ways, there are 

 two general types. In one type the veins are parallel or nearly so, 

 and the leaves are called parallel-veined. The elm is an example. 

 In the other type they are branched so that they form a network 

 and are called netted-veined leaves. In netted-veined leaves the 

 two most common kinds are the feather-like, known as the pinnate 

 type, and the palm-like, known as the palmate type. The leaf of 

 the ash tree is an example of the pinnate type, and the geranium 

 leaf is an example of the palmate type. 



Simple and Compound Leaves. Leaves are classed as simple 

 or compound. In simple leaves the blade consists of a single, 

 flat, undivided surface. In compound leaves there are several 

 separate blade-like parts, or leaflets, borne on the same midrib or 

 the same petiole. Most compound leaves are netted-veined. 

 Nearly all leaves of dicotyledonous plants plants whose seeds 

 have two cotyledons, or seed leaves, are netted-veined. With few 

 exceptions, monocotyledonous plants plants whose seeds have 

 one cotyledon bear parallel-veined leaves. 



