22 



STEMS. 



Fig. 12. DATE PALM. 



merits, produced by an indura- 

 tion and a swelling of their 

 tissue; when they are also 

 hollow internally, they are 

 designated under the name of 

 culm or straw. The stems of 

 wheat, barley, and oats are of 

 this kind. 



27. We give the name of 

 stipe to stems which resemble a 

 round column, as large above 

 as below, and crowned with a 

 cluster of leaves or flowers, like 

 the stems of palms (jig- 12). 



28. The stem of all vascular 

 plants is composed of fibres ar* 

 ranged in bundles (fasciculi), 

 or layers, and variously sur- 

 rounded by cellular tissue ; 

 but we observe very great dif- 

 ferences in their structure ; 

 and these variations, which 

 coincide with differences not 

 less important in their mode 

 of growth, have caused vas- 

 cular plants to be divided into 

 two groups; namely, EX'OGENS 

 and EN'DOGENS.* 



* EX'OGENS (Exogenous plants). From the Greek, ex, from, and geinomai, 

 I grow. A term applied to those plants, a transverse slice of whose stem 

 exhibits a central cellular substance or pith, an external cellular and fibrous 

 ring or bark, and an intermediate woody mass, and certain fine lines radiat- 

 ing from the pith to the bark through the wood, and called medullary rays. 

 They are called EX'OGENS, because they add to their wood by successive 

 external additions ; and are the same as what are otherwise called dicoty'- 

 ledons. They constitute one of the primary classes into which the vegetable 

 world is divided, characterized by their leaves being reticulated ; by their 

 stems having a distinct deposition of bark, wood, and pith ; by their em- 

 bryo having two cotyledons ; and by their flowers being usually formed on 

 a quinary type. 



EN'DOGENS (Endogenous plants). From the Greek, endon, within, and 

 geinomai, I grow. One of the primary classes of plants, so called because 

 their stems grow by successive additions to the inside. They are usually 



27. What is a stipe ? 



28. What is the nature of the stem in vascular plants ? 

 lar plants divided ? 



How are vascu- 



