KINDS AN'O FORMS OK LKAVKS. 



43 



HI) 



Exofc'''llillH Stems. 



HI 



reprosetjting a pioco of Flax-stem ma^'iiiCied; and also in Fi^jf. Sr, which shows 

 ti)«! same Htructure in a woody st«'m, namely, in a shoot of Maple of a year old, 

 cut hoth crosswise and lengthwise. 



1 1 6. The dilVerence becomes still more 

 marked in stems more than one year old. 

 During the second yt-ar a new liiycr of 

 wood is formed outside of the lirst one, 

 between it and the liark ; the third year, 

 another layer outside of tlie .second, and so 

 on, a new layer being formed ench ye:ii' 

 outside of tli.'it of the year before. The 



increase is all on the surface, and buries the older wood deeper nnd deeper in the 

 truid<. F(»r this reason such stems are said to be I'.rw/ci/n/is or ()utsi<le-<'rowini' 

 (from two (Ireek words which mean just this), a, new layer Ix'ing added to the 

 wood on the outside each vear as long as the tree or shi-ul) liscs. And so tho 

 oldest wood, or Heart -(vooil, is always in the centre, and the newest and tho 

 freshest, the S<ij)->roniI, at the circumference, just beneath the bark. 



117. The heart-wood is dead, or soon becomes so. 'j'he sap-wood is the only 

 active part; and this, with the inner bark, which is renewed fi-om its inner face 

 every year, is all of the trunk that is concerned in the life and growth of the tree. 



118. Plants with exogenous or outside-growing stems, especially those tliat 

 . live year after year-, almost always branch freely. All conunon shrubs and trees 



of the exogenous class make a new set of branches every year, and so present an 

 appearance very dilTerent from that of most of those of the endogenous or inside- 

 growing class. 



§ 3. Of Leaves. 



119. Leaves exhibit an almost endless vari(>ty of forms in different plants; 

 and their forms afford easy marks for distinguishing one species from another. 

 So the different shapes of leaves are classified and named very pai'ticnlarly, — • 

 which is a great convenience in describing plants, as it enables a botanist to 

 give a correct idea of almost any leaf in one or two words. We proceed to 

 notice some of the pi-incipal kinds. 



120. Their Parts, A leaf with all its parts complete has a Blade, a Fnofsfall', 

 and a pair of Stipules at the base of the footstalk. Fig. 82 shows all three parts 



