194 '^HE PLANT WORLD. 



esting and beautiful of our flowering plants. It is not restricted 

 to the tropics, but reaches its greatest perfection there. Bletia 

 ncrccitiida is said to have been sent from the West Indies to Eng- 

 land as early as 1731 and since then, from time to time, various 

 species of orchids have been sent by officers and missionaries to 

 northern countries where they have been grown in glass houses. 

 Their value as ornamental plants was early recognized and men 

 were tempted by the high prices to take great risks of health and 

 life in exploration in search of them. One of the most interesting 

 lines of study has been the special devices of these plants for in- 

 sect pollination. Aside from their ornamental value the only one 

 of commercial importance is the J\inil/a plaiii folia or the vanilla 

 plant of Mexico which is now cultivated more or less in tropical 

 America. 



MICROSCOPIC AQUATIC PLANTS AND THEIR 

 PLACE IN NATURE.* 



Every piece of water, besides containing large plants and ani- 

 mals which are readily visible to the naked eye. harbours a more 

 or less considerable number of minute forms, which pervade all 

 the lavers of the water in varving amount, and collectivelv con- 

 stitute the plankton or pelagic life. The most important difl:'er- 

 ence between the plankton and the remaining flora and fauna of 

 our waters lies in the fact that all the organisms which compose 

 it are free-floating during the greater part of their life. Prac- 

 tically all the pelagic plants belong to the group of the algae, 

 and their minute size, of course, suits them well to a floating 

 existence. A certain number of them are motile (c. g., Volvox, 

 Goniimi, Paiidorina, etc.), and these are able actively to main- 

 tain themselves in their position in the water ; but the large ma- 

 jority are non-motile, and all these forms are slightly heavier 

 than water, and consecjuently tend to sink ; they develop diverse 

 mechanisms, by means of which their power of flotation is in- 

 creased. The most important of these are : assumption of a flat 

 plate-like shape (Pediastniiii, Mcrismopedia, many desmids) ; 

 development of numerous delicate processes from the body of the 



* Abstract of a lecture on " The Microscopic Plants of our Water," deliv- 

 ered before the London Institution on February ist In- Dr. F. E. Fritsch. 



