FUNCTION.] ENDOGENS. 141 



ledon : it is composed of a bundle or bundles of vascular tissue 

 encased in pleurenchym, surrounded on all sides with pith, 

 or, which is the same thing, parenchym. Now suppose a 

 number of these petioles to be separated from their blades, 

 and to be tied in a bunch parallel with each other, and, by 

 lateral pressure, to be squeezed so closely together that their 

 surfaces touch each other accurately, except at the circum- 

 ference of the bunch ; if a transverse section of these be 

 made, it will exhibit the same mixture of bundles of woody 

 tissue and parenchym, and the same absence of distinction 

 between pith, wood, and bark, which has been noticed in the 

 'corm, or first plate, of monocotyledons. 



As soon as the plate has arrived at the necessary diameter, 

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

 which were before at its circumference, and carrying 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 afterwards 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, 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 

 obliterated, and appearing among the bundles it formerly 

 surrounded, like the interstices around the minute pebbles 

 of a mosaic gem. 



