MN1UM SPHAGNUM. 231 



most bear on their edge, at nearly equal distances, one- to two- 

 celled, sharply tapering, teeth. Cross-sections through the leaves 

 are obtained at the same time with the cross-sections of the stem. 

 If it is desired to cut cross-sections of leaves by themselves, which, 

 from their small thickness, is no light task, it can be much facili- 

 tated if a considerable number of leaves are stuck together with 

 glycerine-gum, and, without waiting for the gum to dry, the object, 

 thus made thicker, is cut between elder-pith. 1 The cross-sections 

 are then laid in water, which at once dissolves out the gum. This 

 method can be used at all times, when it is desired to obtain cross- 

 sections of very thin surfaces. Upon these cross-sections of our 

 moss-leaf, we can determine that the lamina is one-cell thick ; the 

 cells at the seam-like margin of the leaf are strongly thickened. 

 The midrib projects more strongly from the under than the upper 

 surface of the leaf. In its midst, somewhat nearer the under side, 

 lies a string of thin-walled cells, in which we again recognise the 

 conducting bundle which we previously saw in the cortex. This 

 thin- walled string is supported on its under side by some strongly- 

 thickened narrow cells. 



A withered plant, with the lower cut surface of its stem placed 

 in water, remains withered ; but becomes, on the other hand, 

 rapidly turgid if it is immersed with its leaves in water. The 

 absorption of water through the leaves is here, therefore, very 

 active, and that through the stem is very inactive. 



Stem of Sphagnum. The structure of the stem of the Bog- 

 mosses offers special peculiarities. We prepare cross-sections of 

 the stem of Sphagnum acutifolium. These cross-sections (Fig. 92) 

 show us a broad central cylinder, which in its interior is composed 

 of broad, somewhat collenchymatously-thickened cells ; towards 

 the periphery its cells become gradually narrower, and, in the 

 outermost layers, are coloured yellow-brown (sk). A special 

 conducting bundle is not present in the interior of this cylinder. 

 It is surrounded by a large-celled outer cortex of usually three 

 layers of cells. The elements of this impinge directly upon the 

 narrow, yellow-brown cells of the inner cylinder. They are dis- 

 tinguished by large round or oval holes (pores) and delicate spiral 

 bands. These pores (I) are easy to see, and that they directly join 



1 Good cross-sections of leaves can also be obtained without preparation 

 by cutting through the crowded leaves at the apex (bud) of the stem of an 

 actively-growing moss, such as Mnium undulatum, M. hornum, or a Poly- 

 trichiun. [En.] 



