30 THE GARDENER. [Jan. 



medullary rays, and is in direct communication with the leaf-buds 

 and the veins of the leaves ; and by it oxygen is carried upwards, 

 liberated by the decomposition of carbonic acid or of water, and con- 

 ducts it into the leaves. 



The Wood lies upon the medullary sheath, and consists of concen- 

 tric layers ; is formed by the successive deposit of organised matter 

 descending from the buds, and by the interposition of the medullary 

 system connecting the pith and the bark. New wood is formed 

 annually and deposited between the external surface of the woody 

 skeleton and the inner surface of the liber. Therefore the central 

 wood is the oldest and firmest, and necessarily the most mature and 

 permanent. Our forest-trees supply examples of this kind of growth, 

 and the age of a tree may be known by the number of concentric 

 layers. A layer is the produce of one year's growth in countries 

 having a winter and summer ; but this rule is of uncertain application, 

 as in some tropical countries, owing to the very small and sometimes 

 no period of rest, more than one is formed. 



The Medullary Rays consist of cylinders or compressed parallel- 

 ograms of cellular tissue belonging to the medullary system : they 

 connect together the tissue of the trunk, maintaining a communication 

 between the centre and circumference ; act as braces to the woody and 

 vasiform tissue of the wood; convey secreted matter horizontally from 

 the bark to the heart-wood ; and generate adventitious leaf-buds. 



The stem of Eyidogem offers no absolute distinction of pith, medul- 

 lary rays, wood, and bark. Endogenous plants deposit a layer of 

 wood internally and towards the centre. The external cylinder is 

 consequently the oldest and first formed, and therefore the exterior is 

 the hardest or most indurated, as is also the lowest part of the trunk ; 

 and, it may be repeated, the stem is thus gradually built up by a pro- 

 longation of the fibres of the leaves that are deflected at a specific angle 

 towards the centre. From this view of the case it must appear that 

 the extension in length of Endogens bears no ratio of proportion to 

 their diameter, as is exemplified in the case of Bamboos or the Palm. 



In what are called Dictyogens, the stem has the structure of 

 ExogenSj the root that of Endogens — Smilax is an example. 



XO. IV. — LEAVES. 



Leaves, when perfect and fully developed in flowering plants, consist of two 

 parts : the lamina (limb), or disk — and the petiole, or foot-stalk ; the latter in 

 many cases being articulated or jointed with the branch or stem, so as to be 

 readily detached without laceration when the leaf begins to decay. Leaves 

 originate around the growing apex of the stem— they are never terminal 

 organs — and are expansions of the bark immediately below the origin of 

 regular leaf-buds, and appendages of the axis. They are sometimes opposite, 

 as in labiate plants, such as Coleus and Wood Betony ; alternate, as in Ivy and 

 the Common Garden Pea ; verticillate, as in the common Bedstraw (Galium), 

 &c. As I mentioned before, petioles are foot- stalks, and the petiole is the 



