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, like 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. 
