104 INTRODUCTION TO CEYPTOGAMIC BOTANY. 



Ghordaria filum. The siliceous cases of some of the Diato- 

 maceoB are vakiable test- objects for microscopes, and are used 

 for polishing. 



85. A few words on the mode of preserving Algse for the 

 herbarium may not be amiss in a treatise like the present. 

 Few objects are more beautiful when well preserved ; but a 

 great deal depends upon neatness of manipulation. The fresh 

 water Algas, for the most part, do not make specimen's 

 beautiful to the eye, and the distinguishing characters of very 

 many can only be retained by mounting small specimens in 

 fluid, according to the method which will be described at ihe 

 end of the volume. Useful specimens of many, however, may 

 be preserved for the herbarium by floating them in water and 

 slipping paper beneath them, or by simply drying with as little 

 pressure as is needful to secure neatness and easy preservation 

 in the herbarium. The gelatinous species will not, however, 

 admit of much pressure, and it is often necessary to dry the 

 specimens partially before they are at all compressed. The great 

 difiiculty to contend against in the larger olivaceous Algae is their 

 chemical constitution, in consequence of which efilorescence is 

 apt to take place, and dampness to ensue, which causes mould, 

 and spoils the beauty of the specimens. It is perhaps impos- 

 sible to avoid these inconveniences altogether ; but after the 

 specimens have been washed with fresh water to remove any 

 salt which may adhere to them, careful drying between well- 

 dried paper, &c., frequently changed, will, in a great measure, 

 secure the desired end ; the smaller specimens may be pre- 

 served by floating them in water. As regards Florideous 

 Algae, care must be taken that the specimens are not placed 

 longer in fresh water than is absolutely necessary, as it is fatal 

 to the beauty of many. A large portion may be preserved 

 as before, by simply slipping paper under the specimens when 

 floating, taking care that the branches lie in a natural 

 position, and by no means artistically expanded ; and of the 

 larger species, those which are gelatinous must be partially 

 dried before they are submitted to pressure, while others will 

 dry admirably if frequently changed, exactly after the manner 

 of PhfEnogams. In no case should the pressure be extreme, 

 so as to compress the stems unnaturally or to destroy their 



