42 BOTANY. 



roots and circulated along with the sap ; these consist mainly of air, oxygen, 

 and carbonic acid. In the course of the ascent slight changes take place 

 in the sap, the most important, however, being reserved for the leaves to 

 effect. Here the sap is exposed to the influence of light and air, by means 

 of Avhich carbon and hydrogen are fixed, oxygen and watery vapor given off. 

 The sap thus becomes denser and more fitted to the purposes for which it is 

 destined. After thi^ elaboration it commences its descent, passing through 

 the bark, and transmitted latcrall}^ through the cells of the medullary rays. 

 The descending sap, or latex, is sometimes clear and transparent, at other 

 times it is milky or othcrAvisc colored. In this arc contained the peculiar pro- 

 ducts of the plant Avhich are deposited in various situations. A mucilaginous 

 deposit between the bark and the wood results in the formation of an external 

 layer to the latter, and an internal to the former. Starch and gum are depos- 

 ited in the cells of the new layer of alburnum or sap wood, Avhich, in the spring 

 of the year, may be converted into sugar, and by solution in the ascending sap 

 impart to this its sweetness. 



ON THE GEOGRAPHY OF PLANTS. 



Plants are found distributed all over the surfiice of the earth, wherever 

 heat, air, and moisture co-exist ; and the warmer and moister the country, 

 the more vigorous and varied the vegetation. The same soil, however, is 

 not equally favorable to all plants ; this is mainly owing to the fact, that 

 difl"erent species require different inorganic ingredients, as Avell as different 

 amounts of heat, light, and moisture. The mean temperature of a place 

 exerts great influence upon its vegetation, and as this temperature is 

 affected to a certain extent by the latitude and longitude, it becomes possible 

 to establish an intimate relation between geographical localities and 

 particular forms of plants. Besides the j^cographical arrangement of plants, 

 we may also have a physical grouping, according to the physical features 

 of the soil in which they are found. Thus plants may grow in water, salt 

 or fresh, they may be found in sandy soils, in meadows, in vegetable mould, 

 under trees, or on other plants, and even on animals. Recent discoveries 

 of the highest interest, by Dr. Leidy of Philadelphia, have shown that the 

 occurrence of entophyta in animals is perfectly normal. The following 

 arrangement by Balfour, exhibits the general features of a division of plants, 

 according to their station. 



A. Plants Growing in Water, whether Salt or Fresh. 



1. Marine Plants, such as sea-Aveeds, Lavers, &c., Avhich are either buried 



in the ocean or float on its surface ; also, such plants as Rupjna and Zostera. 



In the Sargasso Sea there are floating meadows of Sargasswn hacciferum — 



gulf-weed. This sea extends from 22° to 36° north latitude, and from 25° 



42 



