INTRODUCTION. XI 



greater number of leaves from a stem and examining the remaining 

 ones under the microscope, while still attached. 



It should be borne in mind that the lowest leaves on a stem, 

 and sometimes too on a branch, are often very small or in other 

 ways far from typical, and these should never be selected for 

 examination ; the same objection frequently applies to the 

 uppermost and youngest leaves, which may not be fully developed, 

 though occasionally they are the only ones in which certain 

 structures, of a very fragile nature, can be observed. In the 

 Pleurocarpous mosses the leaves on the branches are usually 

 smaller and less highly developed than the stem-leaves, and 

 unless otherwise mentioned the latter are always the ones 

 described. A similar difference is sometimes, but more rarely, 

 found between the leaves of the fertile stems and those of the 

 barren branches in the Acrocarpous mosses. 



The position of the leaves when moist, and also when dry, is 

 of great importance, and in many species affords a clear and 

 sufficient specific character recognisable at once even in the field. 

 When the leaves are not otherwise described they are to be taken 

 as arranged equally all round the stem (Tab. II. 5) ; in some cases 

 they are regularly distichous, i.e., springing from opposite sides 

 of the stem in two rows, in which case they are usually, though 

 not always, complanate, i.e., flattened out in one plane like the 

 frond of a fern (Tab. II. 1) ; this latter arrangement is even more 

 frequently the case when the leaves are not truly distichous, but 

 spring from all sides of the btem, the flattened or complanate 

 arrangement giving them however a close resemblance to truly 

 distichous leaves (Tab. II. 2). When a leaf instead of spreading 

 out directly from its base is turned towards another side of the 

 stem it is said to be secund ; it usually happens that all the leaves 

 in such cases are turned towards the same side, when they are 

 termed homomallous (Tab. II. 4) ; when, as very frequently occurs, 

 they are also curved, they are said to be falcato-secund (Tab. II. 3). 



The leaves are frequently undulated or plicate ; to observe 

 this in the moist state it is best to separate the leaves and 

 examine them under the microscope, taking care to submit them 

 to as little pressure as possible ; but when it is de*ired to observe 

 this fact in the dry state it is almost invariably better to do so by 

 means of the lens while still attached to the stem. The same 

 remarks apply to the degree of concavity of the leaves, which is 

 sometimes of importance. 



The cell-structure, or areolation, of the leaves is a character 

 of the highest value. It must be remembered that the cells at the 



