64 ECOLOGY Part II 



held within one cubic inch of soil has been estimated to be four-fifths of a mile. 

 Root pressure pushes sap to the top of the tallest trees. It acts under various 

 conditions, in trees of tropical rain forests where there is no evaporation from 

 the leaves, and in trees of temperate climates before the leaves appear in 

 spring. In some parts of our country the maple sugar season is the time of the 

 first great lift of sap from its winter storage in the roots of sugar maples. Root 

 pressure is all-important to plants. Details of the causes of it are complex and 

 not completely understood. Root hairs are the first actors in root pressure 

 because they carry on the absorption of water from the soil. About one-third 

 of the pressure is believed to be osmotic and two-thirds metabolic, that is, due 

 to respiration and other life processes. 



Reproduction 



Higher plants reproduce asexually and sexually. Some species reproduce 

 more often or exclusively in one way, some in the other. Young strawberry 

 plants develop from creeping stems which grow from the parent; grass plants 

 spread out many sprouts from older plants. The white potato of the dinner 

 table is a food-filled underground stem. When used for planting it is cut into 

 pieces each containing an "eye" or bud from which a new plant grows. In 

 most higher plants both methods of reproduction are common which is never 

 the case in higher animals. A strawberry plant buds forth a new plant; a cat 

 never buds off a kitten. 



The root, leaf, and stem are concerned with the vegetative functions, the 

 intake of food and water, digestion, respiration, and asexual reproduction; the 

 flower with sexual reproduction. In higher plants sexual reproduction is more 

 important than asexual. Any bouquet of flowers — roses, orchids or butter- 

 cups — is a cluster of reproductive organs. Although sexual reproduction differs 

 greatly in detail in plants and animals, its essential features are the same. 



Flower. The flower is the reproductive organ of the plant. The more or less 

 conspicuous parts are the sepals, petals, stamens, and pistil. The latter two are 

 directly and the others only indirectly concerned with the formation of male 

 and female sex cefls and their union in the process of fertilization. Flowers 

 differ greatly in the position and form of the parts and whether male and 

 female cells are borne on the same or different plants of a species. They are 

 often in the same flower as in the diagram (Fig. 4.3). The stamen consists of 

 the stalk supporting the anther and its pollen sacs. When it is mature, the 

 pollen sacs break open and liberate the pollen grains within each of which 

 there are two male sex cells. These are equivalent to the male sex cells (sperm) 

 of animals. The pistil (or pistils) usually consists of a central stalk with a 

 sticky tip, the stigma. At its base is the ovary containing the ovules, the 

 female sex cells equivalent to eggs. The union of the sex cells is brought about 

 in one way or another, such as by the locations of the parts, or by insects. The 



