244 PHYSIOLOGY. BOOK II. 



it begins to lengthen upwards, leaving at its base those leaves 

 that were before at its circumference, and canning upwards 

 with it such as occupied its centre; at the same time, new 

 leaves continue to be generated at the centre, or, as it must 

 now be called, at the apex of the shoot. 



As fresh leaves are developed, they thrust aside to the cir- 

 cumference those which preceded them, and a stem is by 

 degrees produced. Since it has not been formed by additions 

 made to its circumference by each successive leaf, it is not 

 conical, as in dicotyledons ; but, on the contrary, as its in- 

 crease has been at the centre, which has no power to extend 

 its limits, being confined by the circumference which, when 

 once formed, does not afterwai'ds materially alter in dimen- 

 sions, it is, of necessity, cylindrical : and this is one of the 

 marks by which a monocotyledon is often to be known in the 

 absence of other evidence. The centre being but little acted 

 upon by lateral pressure, it remains loose in texture, and, until 

 it becomes very old, does not vary much from the density 

 acquired by it shortly after its formation ; but the tissue of 

 the circumference being continually jammed together by the 

 pressure outwards of the new matter formed in the centre, in 

 course of time becomes a solid mass of woody matter, the 

 cellular tissue once intermingled with it being almost ob- 

 literated, and appearing among the bundles it formerly 

 surrounded, like the interstices around the minute pebbles of 

 a mosaic gem. 



Such is the mode of growth of Palms, and of a great pro- 

 portion of arborescent monocotyledons. But there are others 

 in which this is in some measure departed from. In the 

 common asparagus the shoots produce a number of lateral 

 buds, which all develope and influence its form, as the buds of 

 dicotyledons ; so that the cylindrical figure of monocotyle- 

 dons is exchanged for the conical ; its internal structure is 

 strictly endogenous. In grasses a similar conical figure pre- 

 vails, and for the same reason ; but they have this additional 

 peculiarity, that their stem, in consequence of the great rapi- 

 pidity of its growth, is fistular, with transverse phragmata at 

 its nodes. It is not certain whether the subsequent internal 

 growth of the stem is ever sufficient to fill up the central 



