80 VEGETABLE PHYSIOLOGY. 



during vegetation at the expense of the alburnum ; while on the 

 opposite side towards the bark, the alburnum increases in about an 

 equal proportion : so that in our climates, the alburnum grows each 

 year from a new concentric layer ; but in tropical countries, where 

 the dicotyledonous trees vegetate without interruption, the annual 

 concentric layers are scarcely perceptible. To prove the conver- 

 sion of alburnum into woody tissue, Duhamel inserted a metallic 

 wire into it in several places. At the end of a few years he found 

 that the wire had become engaged in the proper woody layers. 



The most central zone of the trunk or stem is traversed by the 

 medullary canal or sheath, which is usually filled with the pith, a 

 diaphanous spongy matter, consisting almost entirely of cellular 

 tissue. 



The pith sends ramifications towards the external parts of the 

 trunk. Its use is not exactly determined ; and notwithstanding the 

 high purposes ascribed to it by some physiologists, we have many 

 reasons for believing that its functions are not of great importance. 

 Experiment proves, in fact, that the pith may be removed from 

 young trees without killing them, without even stopping their 

 growth. One of the least unquestionable offices assigned to the 

 pith, is that of its being a reservoir for moisture with which it sup- 

 plies the plant in times of drought, and when the ground does not 

 furnish a sufficient quantity of water. 



The internal structure and progressive development of the stem 

 of monocotyledonous plants differ essentially from those which 

 we have just been describing in connection with dicotyledonous 

 plants. 



If a perpendicular transverse section of the trunk of a palm-tree 

 be examined, the same arrangement of zones which is observed in 

 the dicotyledonous plants of our climates will not be perceived. 

 The regions of the outer bark, of the liber or true bark, of the al- 

 burnum, and of the wood, forming so many concentric circles round 

 a canal which is their common centre, are no longer distinguishable. 

 The trunk of the palm-tree presents a more homogeneous appearance. 

 The pith is disposed through the whole substance of the stem, and 

 the woody tissue, presenting a fibrous texture disposed longitudinal- 

 ly, is found intimately mixed, or felted, as it were, with the medul- 

 lary substance. The bark, if there be any, is always very indis- 

 tinct ; sometimes reduced to a simple epidermis, it is with difficulty 

 distinguished from other parts of the trunk. In the beginning, and 

 when it first appears above the ground, a palm-tree puts forth a sys- 

 tem of leaves, the adhering extremities of which are attached in the 

 same plane, and usually surround the neck of the root. At the 

 second shoot, a system similar to the preceding one appears, which 

 throws off the outside leaves, and interrupts their power of vege- 

 tating. These leaves wither, bend towards the earth and fall off, 

 leaving a projecting circular ring on the stem, the only vestige of 

 their existence. The same phenomenon takes place periodically. 

 In the centre of the crown of leaves or branches, which terminates 

 tlie palm-tree plant, a bud appears which is at first small and blanciv 



