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 



