/,<■«/ Wtrialiou Us ILvtcnt and Significance. 51 



The youiij,^ plant is no exception to this law. The first pair 

 of leaves above the cotyledons are regarded as typical of the 

 ancestral forms; these little seedlings bring with them repre- 

 sentatives of bygone ages. The past and the present clasp 

 hands, as it were, over the intervening years, through the 

 seedling and the parent tree. 



The variation of every leaf is probably initiated in the 

 embryo bud. If the variation proves useful to the plant, 

 natural selection takes it in hand, and the next generation of 

 leaves becomes heir to each advance made ; in the course of 

 ages, the modification extends to the seed-germs them.selves. 

 The embryo seed-leaves are also advancing and represent a 

 higher type. The parent plant, or tree, may become modi- 

 fied, and in this manner the difference betw^een the ancestral 

 type and the present would be constantly augmented. 



The leaves of plants are short-lived ; but each year gives 

 them a fresh start from the bud, crowned with the inheritance 

 of all previous time, and even though "the mills of the gods 

 grind slowly," the outcome is variation from ancient forms. 

 The adaptation of plants to changing conditions, necessitates 

 variation of foliage. They must be able to obtain or appro- 

 priate food from the air; must secure to themselves all the 

 sunlight po.ssible ; mu.st be able to withstand wind and storm. 

 Each of these three requisites is more readily attainable if 

 the leaves are lobed, divided, or small, as in compound or 

 pinnate leaves. It is plainly evident to anyone that the air 

 can circulate more freely about a plant which has finely 

 divided leaves; that it flows over and through them in such 

 manner as to come in contact, not only with the outer, more 

 exposed leaves, but with each lobe or leaflet of the entire 

 plant. The sun light also penetrates through the whole mass 

 of foliage more perfectly if the leaves are finely divided. 

 The common horse-radish, Nasturtium Armoracia, may be 

 taken as an illustration of the variation of foliage consequent 

 upon changing conditions. Every one is familiar with the 

 large, thrifty, vigorous leaves of this common plant, but, per- 

 haps, not every one has observed that in the late vSummer a 

 manifest change is initiated in the foliage. The rank growth 

 of the plant has produced a mass of leaves, which are crowded 

 so closely together that the air circulates but sluggishly 



