Plants are distinguished from animals by the absence of a stomach, a 

 permanent or extemporized internal cavity for the reception of liquid or 

 solid food. Though a very few plants have hollow leaves or utricles, or 

 irritable leaf hairs which close around an insect lighting on them, such 

 plants still depend, for their nourishment, mainly on inhaled carbonic acid 

 and liquids absorbed by external roots. Plants are also chemically char- 

 acterized by having their cell walls constituted of cellulose, a substance 

 containing no nitrogen. 



The following table is made up of alternative distinctions successively 

 pointing out to the student which of two or more paths he must follow 

 to determine the natural order of the specimen in hand. 



If no name or number follows the right description, he should go 

 directly on to the next set; if a number follows the correct description, 

 he should turn to the reference given ; and the comparisons should thus 

 be continued till the name of the order is found. 



The small figures after the name of a natural order indicate the page 

 of Gray's " Field, Forest, and Garden Botany " on which that order is 

 described. But sometimes Gray recognizes as sub-orders what others 

 reckon as orders, and in such cases his description accompanies a dif- 

 ferent name from the one here used. 



When no figures follow the name of a phanerogamous order, no spe- 

 cies of the order is known to grow wild in the northern United States. 





