40 TALKS AFIELD. 



and to form ragged ridges running length- 

 wise the trunk. The interior of exogenous 

 stems is occupied by a pith (Fig. 44), and 

 from this pith lines radiate in all directions. 

 These lines are the medullary rays. The 

 interior dark portion is the heart-wood, and 

 the outer light portion the sap-wood. The 

 stems of our exogenous herbs increase in es- 

 sentially the same manner as the trunks of 

 trees. 



It is a singular fact that there are pecul- 

 iarities of the seeds and of the flowers of en- 

 dogenous and of exogenous plants which dis- 

 tinguish the two groups as readily as does 

 the manner of growth. It will be remem- 

 bered that in our study of the bean on page 

 2 we discovered two seed-leaves or cotyle- 

 dons. It is found that the seeds of all en- 

 dogens contain but one cotyledon, while 

 those of exogens contain two or more. The 

 endogens are therefore often styled Mono- 

 cotyledons and the exogens Dicotyledons. 

 The parts of the flower in the endogens are 

 usually in threes or in multiples of three : 

 that is, there are three, or six, or nine sepals 

 and petals and stamens and pistils, or some 

 higher number which is a multiple of three. 

 It is not necessary that these organs be all 



