STRUCTURE, 
13 
which is formed of the turned-back margin of fertile 
contracted fronds. 
The spores are extremely minute, roundish in form, 
crowded into their case without any order of arrange- 
ment. The forms of seed afford another distinction by 
which the leading groups of plants are distinguished. 
The exogens, or outside-growers, have seeds composed 
of two cotyledons, between which the embryo is dis- 
posed, consisting of the radicle , or rooting-point, and 
the plumule , or growing-point. The endogens, or inside- 
growers, have but one cotyledon, but the points of 
growth and rooting are clearly determined. The acro- 
gens, or summit-growers, have no cotyledons and no 
determinate point of growth or rooting ; they germinate 
indiscriminately from any point. 
A fern-spore consists of mere cellular tissue. The first 
change which the spore undergoes is the bursting of the 
outer covering, and the protrusion of the delicate inner 
membrane. After a short period this membrane di- 
vides, and produces a thallus of two cells. The second 
cell produces another cell, and so on until five or six are 
formed. Soon after the production of the second cell 
chlorophyll appears, and the plant assumes the form of 
a Liverwort. From one of the cells root-fibres diverge, 
and strike down firmly into the ground. Presently new 
fronds arise, each one partaking more and more of the 
parent form, and the rudimentary thallus-like frond dis- 
appears. 
Ferns are the most highly-developed of acrogenous 
plants. In the algae and fungi the whole plant consists 
of cellular tissue, but in the fern group both woody 
matter and vascular tissue are found. It is principally 
in the caudex that the woody matter is found, and it 
