262 A TEXTBOOK OF GENERAL BOTANY 
single-celled plants which not only lack sexuality but which do 
not have a well-defined nucleus, and in which the photosynthetic 
coloring matter is diffused through the protoplasm instead of 
being contained in special plastids (Fig. 312). In somewhat more 
highly developed types the whole plant consists of either a single 
cell or a group of similar cells with well-developed nuclei and 
chloroplastids. Between this simple vegetative structure and the 
differentiation. characteristic of flowering plants there are many 
gradations. The various groups of 
plants will be considered in subse- 
quent chapters. 
In animals we find functionless 
structures which can best be ex- 
plained on the ground that they 
have been inherited from ancestral 
| forms in which they were of use. 
Fra. 240. Leaves of a speciesof Phe human appendix is a well- 
Mencia known example. In human beings 
On the left is the first leaf afterthe this structure is rudimentary and 
cotyledons ; on the right, asome- apparently serves no useful pur- 
what older leaf in which the petiole pose, while in some lower animals 
is flattened and leaflike and takes * . 
the place of the leaflets. (x1) 1t 18 a well-developed and useful 
organ. 
The water plants from which the present land plants appear 
to have developed have motile sperms which swim in the water. 
Such motile sperms are possessed also by the simpler of the land 
plants, such as ferns and mosses. The simplest seed plants like- 
wise have motile sperms, although these plants are fertilized by 
pollen tubes in much the same way as are flowering plants 
(Fig. 468). The presence of motile organs on the sperms of 
these seed plants can be explained best by assuming that these 
organs were derived from aquatic ancestors. 
In the embryology of many animals, and to a less extent in the 
development of certain plants, there are characters which disap- 
pear in the adult and which resemble characters of more prinitive 
ancestors. For example, ordinary leaves occur on the seedlings 

