294: R. BIIAITIIWAITE ON THE HISTOLOGY OF PLANTS. 



already described. All three forms, annular, spiral and reticular, 

 are found near the pith in the youngest part of the Vascular 

 bundle, while in the older parts, only bordered porous vessels 

 occur. They are most numerous, either single or grouped, in the 

 inner and middle portions of the annual ring, while in the outer 

 part or autumn wood they are often absent, or scattered singly 

 {Fraxinus), or forming mixed groups with the other two kinds 

 {Quercus) ; the vessels of the spring wood have a much wider 

 lumen than those formed in autumn. 



In Viburnmn, Carpinus, Betula, &c., only scalariform vessels are 

 found, while in Lonicera caprifolium, Fagus sylvatica, and Platanus^ 

 both scalariform and porose occur together. In old vessels a 

 peculiar cell formation takes place in the interior, so that the whole 

 becomes filled with roundish thin-walled cells, and these appear to 

 proceed from the surrounding parenchyma, which grows inward 

 through the pores ; we see this in Nerium, Cucurhita, Vitis, 

 Eobinia, &c. 



B. The hast-part of the Vascular bundle. — Bast fibres. — These 

 also proceed from the cambium at the same time as the wood, but 

 from that part next the bark ; their form is spindle-shaped, and 

 only in rare cases truncated at point as in some Cactece and 

 Euphorbia. Branched bast cells occur especially in Asdepiads 

 and Apocynece, as in the leaves of Hoy a and Vinca, those of the 

 stem being mostly unbranched and with narrow lumen ; partitioned 

 bast cells occur in the vine, some Cactece and Sambucus racemosa. 



Except in a few plants ( Urtica dioica, Vitis, Ficus elastica and the 

 Solanacece), the thickening of the wall is very marked, and shows 

 distinct lamination ; but this secondary thickening is frequently 

 perforated by simple or branched pore canals, as in Clematis and 

 Brtigmansia, the pores being unbordered, and spiral thickening 

 also occurs ; in most cases the whole of the thickening layers are 

 completely lignified. 



The arrangement of bast cells in the Vascular bundle is very 

 variable; in many plants, as in the Beech and Birch, they form 

 connected groups of greater or less extent, which stand at the 

 edge of the bast part next the bark and laterally are only separated 

 by the medullary rays ; in others, as Clematis^ Vitis, &c., they 

 form radially arranged rows of one or more cells, or as in Tilia 

 and Begonia similar groups of larger size ; in Hoya, Asclepias, &c., 

 they are scattered in the bast part. In the young state the con- 

 tents are protoplasmic granular sap, but afterwards air. 



