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THE PERISTOHE V. 



By a. J. Grout. 



In my third article on this topic I called attention to the fact that the 

 peristomes of the Buxbaumiaceae are somewhat intermediate between the 

 Arthrodont and Nematodont types. In the Encalyptaceae also there are 

 some interesting transitional stages which, perhaps, will be better under- 

 stood after we consider the method of formation of the Arthrodont peristome 

 in detail. I have chosen Mfimm hoi-num to illustrate this article because 

 Schimper's illustrations of this species serves my purpose so well. 



Fig. I. Transverse section 

 through the upper part of a nearly 

 ripe mature capsule of Mnitim 

 hornum. This figure shows th^ 

 upper portion of the archesporium 

 with several spores: the columella 

 occupies the upper portion inside 

 the peristome; the annulus is 

 shown at the junction of the 

 operculum with the mouth of the 

 capsule ; x 120. 



The basis of the Arthrodont 

 peristome, both double (Dipio- 

 lepideae) and single (Aplolepidese) 

 is a layer of sixteen large cells 

 lying just underneath the oper- 

 culum and shown in cross section 

 in Fig. 3. This layer we will call 

 the peristomial layer for conveni- 

 ence. In Mnium hormcvi and 

 nearly all the double peristomed 

 Fig. I. mosses except the Funariaceae 



the teeth of the outer peristome are formed by the thickening of the outer 

 walls of the cells, and the inner peristome by a much less pronounced thick- 

 ening of the inner walls. The thickening laid down on the outer walls is 

 continued along the horizontal walls which separate each tier of cells, form- 

 the projection (trabeculae) on the inner faces or the teeth, Fig. 5 and Fig. 2, b. 

 These trabeculae and the walls thus thickened form the transverse lines 

 separating the joints or articulations in the teeth of Mnium, Bryum, Hyp- 

 711(711, etc. The layer of cells next outside the peristomial layer is composed 

 of twice as many cells, and the median ziz-zag line of the peristome teeth 

 marks the intersection of vertical radial wall of these cells with the outer 

 wall of the peristomial layer, the alternating walls coming at the boundary 

 line between the teeth and consequently not leaving any traces. (Fig. 3). 

 A reference to Plate I. of this volume of The Bryologist, Figs. 15 and 17, 

 will be very helpful in understanding this matter. A description of these fig- 



