134 CELLS AND TISSUES 



are the chief reproductive cells, all the tissues of the stamens and 

 pistils are related to fertilization, which is the chief feature in 

 sexual reproduction. 



In plants like Ferns, Mosses, and Algae, where there are no 

 flowers, the sex cells are commonly borne in special organs, called 

 sex organs, which are so constructed as to favor fertilization. 



Summary of the Cell and Tissues. — The cell is the unit of 

 plant and animal life. It contains the living substance, known 

 as protoplasm, which is usually enclosed in a cell wall. The pro- 

 toplasm is composed of a nucleus and cytoplasm. Cells receive 

 water, food, and mineral elements through osmosis and ordi- 

 nary diffusion, and obtain energy through respiration. Cells 

 multiply by division. Cell multiplication is accompanied by 

 cell modifications which result in the differentiation of the cells 

 into tissues. Some tissues consist of only the modified cell 

 walls, the protoplasm having died and disappeared after the 

 modifications of the walls are complete. The higher plants have 

 many tissues, each of which has one or more functions. The 

 meristematic tissues enable the plant to continue growing; epi- 

 dermal and cork tissues protect the plant from drying and from 

 attacks of destructive organisms ; collenchyma tissue, bast fibers, 

 and wood fibers enable the plant to support itself in a favor- 

 able position in spite of the force of gravity and winds; vas- 

 cular bundles, hke the circulatory system of animals, supply the 

 other tissues with materials; the absorbing tissues take from the 

 soil, or other substrata, the water and dissolved substances which 

 the plant must have; the storage tissues hold the water or food 

 in reserve for future use. The stored water is used during dry 

 seasons, and the stored food is used for the growth of new plants, 

 as in case of seeds, tubers, etc., or for the new growth of leaves 

 and flowers at the end of a dormant period, as in case of trees. 

 The food-making tissues furnish the food which all parts of the 

 plant must have. Secretory tissues assist in cross-pollination 

 by providing secretions which attract insects, furnish nectar from 

 which honey is made, and give us many other products, such as 

 resin, turpentine, etc. 



