196 STRUCTURE AND LIFE HISTORIES 
As the leaves mature, part of their cells increase greatly 
in size, and the protoplasm becomes entirely transformed 
into thickenings of the cell-walls, leaving the cells quite 
empty of everything but air and water. These large, 
empty cells greatly mask the smaller ones containing 
chlorophyll, and this accounts for the pale color of . the 
plants. The walls of the empty cells are commonly per- 
forated with several pores. A similar type of cell is also 
developed in the outer layers of the stem (Fig. 141). 
These cells are extremely hygroscopic, and absorb water 
rapidly and in large quantities, so that the entire living 
plant is usually thoroughly saturated with water. On 
account of its sponge-like nature, sphagnum is much used 
by florists in packing plants for shipment, and in other 
ways. 
179. Sterile and Fertile Branches. Two kinds of 
branches occur: sterile and fertile. The organs of repro- 
duction (antheridia and archegonia) occur only on the 
fertile branches, but antheridia and archegonia never on 
the same branch. In some species they occur on separate 
plants (dioecious two households) ; in other species on the 
same plant (monoecious one household). 
180. Antheridial Branches. The male branches, or 
antheridiophores, bear leaves that vary in color from 
green to yellow and red, and the antheridia occur in the 
axils of these leaves (Fig. 142). They consist of a rela- 
tively long stalk, composed of four rows of cells, or less, 
and bearing, at maturity, the globular capsule containing 
the sperms (Fig. 143). The sperms are coiled, with about 
two complete turns, and bear two long thread-like cilia at 
their anterior end. In locomotion the end bearing the cilia 
precedes. At the opposite or posterior end occurs a small 
