128 



ANATOMY. 



(F) remaining isolated and scattered through the medullary system (M) without any 

 medullary rays of muriform tissue. Here there is no symmetrical arrangement ; the 

 bundles are scattered throughout the pith, and may multiply without being impeded 

 by lateral pressure ; further, each remains simple ; at no period does it develop 

 between its bark and wood systems a layer of cambium destined to form new 

 bundles. In dicotyledons, on the contrary, the bundles are pressed into zones 

 from the first year, and their wcod and bark systems being concentric, they can 

 only multiply by fresh wood and bark bundles being formed between 

 them. The consequence of this arrangement of the fibro- vascular 

 bundles is, that in dicotyledons the stem is hardest towards the centre, 

 whilst in monocotyledons the stem is hardest towards the circum- 

 ference; as is very apparent in the woody (fig. 692), and even in 

 the herbaceous stems of monocotyledons. In a longitudinal section 

 of a woody (fig. 693) or herbaceous (fig. 694) monocotyledonous stem, 



those differences are still 

 more apparent ; starting 

 from the insertion of a 

 leaf, each bundle descends 

 at first obliquely inwards, 

 then vertically, then again 

 obliquely outwards ; cros- 

 sing in its path all the 

 bundles which have origi- 

 nated below it, and are 

 hence older than itself, and 

 ending by taking up a 



693. Theoretical section of a position Outside of them stem c* 



Palni stem. * _. , , - vertically. 



all. In dicotyledons also, 

 the youngest bundles are the outermost, those of the same age follow nearly 

 parallel ; but whereas in their courses they unite so as to form a cylinder, in 

 monocotyledons they diverge below and converge above. The composition of mono- 

 cotyledonous bundles also differs in different parts of their course, the wood system 

 predominating over the cortical in the upper part, where it descends obliquely 

 inwards, the cortical system predominating in the lower part, where it descends 

 obliquely outwards, and finally the cortical system alone being developed where the 

 bundle reaches the periphery. Here the bundle becomes more slender, and divides 

 into thread-like branches, which interlace with those of the neighbouring bundles, 

 and form together, within the cellular periphery, a layer of fibres comparable, 

 according to many botanists, with a liber zone. 



It is obvious that these fibro-vascular bundles, being composed of different 

 elements at different heights, and becoming so slender towards the periphery, must 

 appear very dissimilar in a horizontal cut of the stem ; the scattered small bundles 

 with large vessels, which occupy the middle of the stem, are the upper portions of 

 bundles in which that which we have called the wood system (though it is rather 



692. Palm. 

 Stem cut vertically. 



