THE ROOTS OF PLANTS. 



ture, through which the vascular tissue may pass from the leaves towards the roots 

 and secondly, to contain starch, oftentimes of great purity and abundance, as in the 



stem of the sago palm (Sagus 



Rumphii), 



The points in the external 



characters of endogenous stems, 



which are peculiar to these 



structures, are the following : 

 First. As a rule the stem 



does not give off branches, but 



proceeds either singly or dicho- 



tomously (divided into two) 



from the base to the apex. 



Fig. 146. Horizontal section of 

 the Sugar Cane (Saccharinum 

 officinale) showing the cellular 

 structure or pith at <, and the 

 intervening bundles of vascu- 

 lar tissue at c. 



Fig. 147. A section of the 

 stem of the common Iteeu 

 (Phragmites], showing at 

 a the central vacuity, and 

 at b the solid stem, with 

 the bundles of woody 

 tissue. 



Secondly. The leaves are 

 therefore found only at the 

 head of the stem, and surround 

 it by numerous insertions, ar- 

 ranged above each other in 

 a spiral manner. They are 

 usually of large size, and thus compensate for their paucity in numbers. When their duty 

 has been performed, they wither ai.cj aecay upon the stem, or ultimately fall from it, 

 leaving a mark called a cicatrix to radicate the points to which they were formerly 

 attached. The succession of leaves takes place from below upwards, and thereby a 

 palm of some years growth presents a series of cicatrices. As the age of the tree 

 cannot be determined by the number of the concentric circles of the wood, it is inferred 

 from the number of rows of cicatrices, which indicate the successive seasonal formation 

 of leaves. 



Thirdly. From the above reasons, the stem is not conical, but cylindrical, and is tall 

 in proportion to its diameter. Not that it is cylindrical, as opposed to conical, to the 

 very apex of the plant, (for the very apex has not so large a diameter as the inferior 

 part,) but since the reduction in diameter is somewhat sudden, and is found only very 

 near to the terminal part of the stem, it is more truthful to state that the figure of the 

 stem is a truncated cylinder than a cone. 



In all these respects the stems of endogens are very unlike the stems of exogens. 

 The Descending Axis or Root. Having now completed our account of stems, 

 it will be more convenient to state the little which may be necessary respecting roots, 

 before we proceed to a description of the numerous and complex structures which are 

 attached to stems. 



We have already stated that the stem and root proceed in the seed from a central 

 spot termed the collum, and that they hence take opposed directions, the stem 

 ascending, the root or radicle descending. This direction of the root is almost uni- 

 versal ; and wherever it is once attained, no power short of the death of the plant can 

 prevent its progress. 



As the root naturally seeks to bury itself in a medium much more dense than itself, 

 it is so formed that its extremity has but a very minute diameter, and at that extremity 

 it is composed of the most delicate structure. The vital process of growth enables it to 

 insinuate itself, without injury, between stones and other resisting substances. Thus 

 the root has naturally a conical figure, with its base opposed to the base of the stem, 



