PREPARATION OF SPECIMENS OF OPUNTIA. 



279 



in this group of plants as in all others. The difficulty of prepara- 

 tion is no excuse for not having them. They are always of more 

 permanent and certain value than living material. 



The most important collection of the genus O/'^nzi/a, and doubt- 

 less of other cacti in this country, is that in the Engelmann Her- 

 barium in the Missouri Botanical Garden, but, aside from this 

 collection, the garden, like other repositories, contains but little 

 material. The Engelmann specimens came to him largely 

 through some of the western surveys and in many cases con- 

 sisted of one or more joints dried in the open air without pres- 



1-.^^ 



^fBsB 



">,^-' 



^W^^l 



;f\J^[- i 



Fig. 49. An arborescent prickly pear, Opiuitia Engelmannii. 



sure. These are stored in boxes, but pressed specimens mounted 

 upon sheets in the ordinary fashion are also numerous. 



There are several ways in which these plants may be preserved 

 and each way has some advantages over the others. On the 

 whole, a specimen of Opiiutia properly prepared and pressed be- 

 tween blotters is the most satisfactory. There are some char- 

 acters which can not be perfectly preserved by this method, it 

 is true, but no herbarium specimen shows living characters per- 

 fectly. In all cases it is an approximation only. 



In many succulent plants boiling before being put in press is 

 advocated but this it not at all satisfactory with Opnntia. In 



