494 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



penetrate the substratum. But these organs are nevertheless effective 

 and, with the watery medium providing almost unlimited possi- 

 bilities for development, practically all stone or otherwise suitable 

 surfaces within the littoral, including dead or living plants, are more 

 or less thickly populated by attached organisms. 



The means of attachment of benthic creatures are many and 

 various, and often have to afford protection against washing away 

 by wave action or currents. They include gelatinous stalks of di- 

 verse structure which are sufficient in quiet water and are found in 

 many Diatoms and in such animals as VorticeUa. Also abundant 

 in both plants and animals are rigid or gelatinous coverings attached 

 to the substratum by tiny stalks or by a broader base. In filamentous 

 Algae the basal cell must bear the entire pull, and so it is often 

 firmly fastened by a lobed attachment-disk which is closely applied 

 to the substratum. In agitated water more resistant attachments 

 are needed and these include thick and shortened gelatinous stalks, 

 flattened thalli broadly attached to the substratum, and gelatinous 

 cushions often reinforced with lime. 



The populations attached to underwater stones and to living plants 

 — such as parts of various Potamogetonaceae and Pontederiaceae — are 

 largely different, as is often evident to the naked eye. Thus on stones 

 and rock surfaces, and often on old pieces of wood, crust-like growths 

 frequently of considerable thickness predominate, whereas on living 

 stems and leaves, as well as on such as are rapidly decomposing, the 

 investment tends to be light and flocculent, often consisting mainly 

 of filamentous Algae. The relative lightness of the investment on 

 living and rapidly decomposing substrata appears to be due largely 

 to their more or less transitory nature, so that they support chiefly 

 colonists that must be quick-growing and short-lived. It also seems 

 to be due in part to chemical changes in the immediate environment, 

 brought about by the living or decomposing ' hosts '. A special 

 community is afforded by those Algae and Lichens that penetrate 

 the substratum and hence live partly within the body of stones, etc. — 

 particularly limestone and snail-shells. 



In the eulittoral, owing to the fluctuations in water-level and the 

 influence of waves and spray, marked changes in conditions often 

 take place o\er \ery small vertical distances, and there may be con- 

 siderable differences in the composition of the communities from 

 spot to spot. Most of the species that are resistant enough to thrive 

 under the rather extreme conditions here obtaining, grow slowly and 

 can develop only on a firm substratum. In small lakes the eulittoral 



