i] BIOLOGICAL CLASSIFICATION 5 



earlier stage. The present writer has freely modified Schenck's 

 scheme, and has carried the sub-division to a further point. The 

 various types met with amongst aquatics are arranged in a linear 

 series for the sake of simplicity ; but this plan is obviously open 

 to the same criticisms as all other linear systems, whether bio- 

 logical or phylogenetic. The following classification is outlined 

 with the utmost brevity, and aims merely at supplying a key to 

 the biological forms encountered. The life-histories of typical 

 plants illustrating the characters of the more important sub- 

 divisions will be considered in some detail in Chapters n x; 

 but the order in which the life-histories are grouped in these 

 chapters has been determined mainly by reasons of natural 

 affinity, and thus bears no close relation to the following 

 scheme. 



(2) BIOLOGICAL CLASSIFICATION OF HYDROPHYTES 

 I. Plants rooted in the soil. 



A. Plants which are essentially terrestrial, but which are 

 capable of living as submerged water plants, though without 

 marked adaptation of the leaves to aquatic life. 



E.g., Achillea ptarmica, L. (Sneezewort). 



Cuscuta alba^ J. and C. Presl (Dodder). 

 Glechoma hederacea^ L. (Ground Ivy). 



B. Plants which are sometimes terrestrial, but sometimes 

 produce submerged leaves differing markedly from the air 

 type. The air leaves are associated with the flowering stage. 



E.g., Certain Umbelliferae, such as Slum latifolium^ L. 

 (Water Parsnip). 



C. Plants which produce three types of leaf, (a) submerged, 

 (ft) floating and (c) aerial, according to the conditions internal 

 or environmental. 



(i) Plants in which the aerial type of leaf is generally 

 associated with the flowering stage. 



E.g., Many Alismaceae, such as Sagittaria sagitti- 

 folia, L. (Arrowhead). 



