64 



DOMESTIC BOTANY. 



number of concentric wheels. This mode of structure is 

 termed exogenous (growing outward), and is the result of 

 all seeds having two seed-leaves; it is further well 

 marked by the veins of the leaves being connected toge- 

 ther in irregular meshes like network, as may be seen in 

 all trees and shrubs, as well as most herbaceous plants 

 in the open air in this country (fig. 13, a, c), the ex- 

 ceptions being the Fir and Yew families, which have free 

 veins. 



Although many plants with reticulated leaves have no 

 apparent stem, the bud or crown (gemmae-corm) from 

 which the leaves spring, is formed on the principle of 

 outward extension, such being the case with many 

 herbs. 



The second mode of stem-formation is the reverse of 

 that above described, the increase taking place by succes- 

 sive development of leaves on the apex of an axis which 

 increases in length. In this case, there is no distinction be- 

 tween bark and wood, consequently no concentric rings, a 

 vertical section showing the increase of new matter to be 

 from the base of the leaves inwards, the whole being homo- 

 geneous. The consequence is, that most tree stems of this 

 division maintain a cylindrical form throughout, increas- 

 ing but little in diameter, that which takes place being due 

 to the outward pressure of additional new matter forming 

 in the interior. In some aloes and scandent palms no in- 

 crease takes place, even although (as in the latter) grow- 

 ing to one hundred or more feet in length. This mode 

 of structure is termed endogenous (growing within), and is 

 the result of all seeds having one seed-leaf ; the structure 

 of the true leaves is also quite distinct from that of 

 the preceding in the veins not being reticulated, but 

 rising from the base of the leaf, running parallel towards 



