246 PRACTICAL LESSONS IN SCIENCE. 



tion of organs, and in the division of labor, which exist so fully 

 in the higher plants. 



As the conditions became more favorable and plants increased 

 in size, a fibre-vascular system became necessary to strengthen 

 and support the plant, and to convey nutrient fluids to all its 

 parts. As plants grew more numerous the supply of light and 

 air became somewhat limited, the fibro-vascular system was 

 strengthened and an erect form was acquired, thus greatly in- 

 creasing their opportunity for these essentials of plant life. 



The change in form and habit resulting from this struggle for 

 light and air, is often very marked. Trees growing singly in the 

 open field are shorter, thicker and have a relatively broader 

 foliage system than trees of the same species growing in a 

 crowded forest. The purslane, which is usually a prostrate creep- 

 ing plant, when crowded by other plants will assume an erect po- 

 sition, rising to seek the light. 



Coincident with this growth of the stem, the leaves were devel- 

 oped from the vascular system, parenchyma, and epidermis, and 

 so arranged as to serve for digestive organs. As the work of 

 digestion or assimilation can only be carried on in the light, we 

 can easily understand that the struggle for light involves the life 

 or death of the plant. This struggle may account for many in- 

 teresting forms of plants; as vines climbing up the stems of stur- 

 dier plants, in whose shadow they could not live, to expand their 

 leaves in the life-giving light above. In some cases climbing 

 plants become air plants, able to derive nourishment from the 

 air, or parasites, living on the juices of the host plant, in either 

 case having no use for a stem connecting with the ground . In 

 this way plants become permanent residents amid the branches 

 of trees, so that a tree often appears to produce several kinds of 

 foliage. Thus the common ivy, sometimes becomes an air plant, 

 gathering nourishment from the air and from its support. 



In some flowers the stamens and pistil or pistils are so ar- 

 ranged that self-fertilization is possible, but in general self-fertili- 

 zation is difficult or impossible, so that most flowers are fertil- 

 ized by pollen from other flowers. The pollen grains are carried 



