THE PROTOPLAST AS HOUSE-BUILDER 33 



known as the pitted or dotted cells, the secondary deposit is spread upon the 

 cell-walls so as to leave little pits, open on the interior side of the cell, and 

 closed at the exterior by the primary cell-wall. These pits have the appear- 

 ance under the microscope of transparent specks (fig. 53). When several 

 dotted cells come together, it often happens that the pits of their con- 

 tiguous walls are coincident ; and the utility of this very beautiful arrange- 

 ment is at once evident : for even after the cells have attained a considerable 

 thickness, they are still permeable to the fluid from without, which is taken 

 in through these little pores and used up by the imprisoned but still living 

 and working protoplasts (figs. 53, 54, 55). 



Phto by] 



FIG. 56. BOG-MOSS (Sphagnum acutifolium). 



[E. Step. 



The Bog-mosses grow in wet hollows where the soil is sour and too poor to maintain mosr plants. The decay oi the 

 older parts, pressed down by the newer growth, results in the formation of peat. COLDER TEMPERATE REGIONS. 



In certain plants of the Cactus order (as Melocactus, Mamillaria, and 

 Opuntia), the wood is entirely composed of short spindle-shaped cells, in 

 which are elegant spiral bands of secondary deposit, looking, as Schleiden 

 neatly expresses it, " like little spiral staircases " (fig. 57). "We call these 

 spiral cells. The large elongated leaf-cells of the Bog-moss (Sphagnum) 

 (fig. 56) and the leaf-cells of many orchideous plants have spiral fibres 

 loosely coiled in their interior; but a better plant than either Orchid or 

 Bog-moss for studying these spirals is the Wild Clary (Salvia verbenaca), a 

 portion of the seed-coat of which makes an extremely interesting object 

 under the microscope. If a very thin slice of the outer coat, moistened with 

 a drop of water, be placed between the glass slides, the delicate fibres will 

 6 



