100 SUMMARY OF CURRENT RESEARCHES RELATINQ TO 



made at the University of Michigan, In the earliest stage the mother- 

 cell of the stoma occurs as a cell of equal rank with the others, but 

 divided into four guard-cells lying side by side, their longitudinal 

 axes being parallel to that of the stem. As the ordinary epidermal 

 cells develope they grow more in length than in bx'eadth, while the 

 stomata grow equally in both directions, and crowd iipon the sur- 

 rounding cells. In the meantime the outer pair of guard-cells arch 

 over the inner pair, and finally completely cover them. When mature 

 the guard-cells are loaded with silica arranged in radiating bars, and 

 usually the inner pair is entirely hidden ; but if a thin cross-section 

 of a mature leaf is obtained, the two pairs of cells can be seen one 

 above the other. In E. arvense the storaata are not, as stated by 

 Sachs, found in the furrows ouly ; iu the leaves they are found, as a 

 rule, invariably on the ridges. In this species the distribution of the 

 stomata on the stem and stem-leaves is alike on the fertile and sterile 

 plants, but on the branches the arrangement is different. On the leaf 

 of the branch there is no central furrow, but its place is taken by a 

 sharp ridge, and the ridges and furrows of the leaf are all continued on 

 the branch. On the leaves the stomata are borne on either side of 

 the ridge, but on the stem they pass down into the furrows, and lie 

 on either side of the lowest part. 



This arrangement of the stomata is peculiar to Equisetum arvense ; 

 in E. limosxim and hyemale they lie, as regards the stem, exclusively 

 in the furrows. 



Muscineae. 



Peristome of Mosses.* — M. Philibert describes the structure of 

 the peristome in various groujjs and species of mosses, and discusses 

 its value as a character to determine genetic affinity. In opposition 

 to the ordinary view that cleistocarpic and gymnostomous forms are 

 the earliest, he believes the earlier structure to be the perfect peri- 

 stome, from which the others have varied by degeneration. Accord- 

 ing to this view the Encalyptaccfe may be regarded as the central point 

 whence all the other forms of moss have diverged; all those which 

 have a peristome nearly identical in structure being probably of a 

 common origin. It is difficult on any other theory to account for the 

 almost complete identity of the peristome in Dicranum, Fissidens, Cam- 

 pylojms, Dicranella, Cynodontium, Trematodon, Dichodontium, and 

 Leucobryum. The same form is found in the pleurocarpous Hypnaceae, 

 and in the acrocarpous Bryaceae, Mniace09, and Bartramiaceae. The 

 peristome can therefore only be used for purposes of classification 

 when present in a comparatively perfect state of development ; when 

 this index fails, recourse must be had to characters derived from the 

 vegetative system. 



In the Arthrodontaceae the exterior peristome has two distinct 

 forms ; either the teeth have a double exterior and a single interior 

 series of plates, or the exterior series is simple, and then the interior 

 series is nearly always double. Philibert proposes the term Aplo- 

 lepidaceae for mosses with an articulated peristome and a simple series 



* Rev. Bryol., xi. (1884) pp. 49-52, 65-72, 81-87. 



