ANATOMY OF TISSUES. 89 
Beginning with the Pteridophytes, and from this class up 
to the highest form, the order is reversed. The plant of the first 
generation, the one bearing the sexual organs, is small, incon- 
spicuous, simple in both morphological and anatomical structure. 
It gives rise to the egg-cell, which, after fertilization, germinates 
and grows into the conspicuous and complex plant. 
The Pteridophytes include several classes of plants varying 
widely in outward form and structure, though similar in respect 
to those organs which are made the basis of classification. Some 
of their general anatomical characters are as follows. The egg- 
cell begins its development by investing itself with a wall. It 
then divides into two cells by a horizontal wall. Further divi- 
sions take place, all the cells remaining meristematic for some 
time. Localization of growth occurs in such a way that in 
nearly all cases a root is developed from the cells originating 
from the lower cell of the spore, while stem and leaves are 
developed from those of the upper half. In some instances the 
lower half of the egg-cell develops another organ called the foot, 
which consists of parenchymatic cells, and whose office is to 
connect the plant with the parent organism and to act as food 
conductor until the young plant is sufficiently developed to care 
for itself. This organ finds its counterpart in all the succeeding 
forms ; but as its office ceases with the independence of the new 
plant, it dies and disappears when no longer needed. 
We have now reached that stage of plant development which 
includes all the organs of the highest form, namely stem, root, 
and leaf. The anatomy of the remaining forms is therefore 
limited to a comparative study of these three organs. 
1 The terms upper and lower here are used in respect to the above mentioned 
horizontal wall. Horizontal here refers to the position of the new plant in ref- 
erence to the thallus or mother plant from which it originated, so this does not 
imply that the axis of the plant is perpendicular to the plane of the earth’s 
surface, 
