o5« NATURAL lUSTOBY OF PLANTS. 



accordiii"" as the openii\2:s of tlie pores are rounded or more or less 

 eloii<^atcd. The medullary rays are numerous, forming very distmet 

 septa, and consist of muriform rectangular cells, much elongated in the 

 radial direction of the stem.' Their walls are very thick, regularly 

 studded witli narrowly areolate perforations. These rays pass dis- 

 tinctly from the wood into the bark, and in transverse section they 

 are seen isolating the divisions of the liber, which is of a characteristic 

 nature in Anonacece. In each of the divisions referred to are several 

 concentric sheets of liber fibres produced in the same year. Each 

 sheet is wholly separated from the two respectively internal and 

 external to it, by a band of cellular tissue.' After several years 

 these alternating bands of prosenchyma are very numerous, becoming 

 narrower as they are more external, so that in transverse section 

 the segments of liber are nearly rectangular, but later on they 

 elonc^ate in the radial direction, and assume the form of a trapezium 

 with the external base very short. Hence results also a defor- 

 mation of the cortical cellular masses continuing the medullary 

 rays of the wood, which become also trapeziums, but with the 

 larger base outside.^ As these external masses enlarge, the cells 

 composing them elongate transversely, but grow very little in 

 the radial direction ; each finally becoming a long curved paral- 

 lelopiped, with its convexity outwards. Their contents are for the 

 most part colourless, but those of several cells bounding the liber 

 bundles on each side usually contain a little chlorophyll. This is 

 very abundant in the true herbaceous layer ; the suber, on the 

 contrary, early becomes brown, and its flattened cells are rapidly 

 pushed out towards the periphery of the bark. Those covering the 

 liber bundles project more at the surface than those answering to 

 the parenchyma between them, thus producing alternate ridges on 

 the surface of the bark, indicating the arrangement of the bundles 



' 1)E .MARTirs says {loc. cit.) tliat the medul- partitions directly continuous with the paren- 



lary rays of the stem of Anona cmssijlora con- chyma of the herbaceous layer. This irregularity 



sist of thick cells, and that the wood is in part is very well marked in the stems of Monodora, 



made up of large pellucid cells, perforated by otherwise constructed as in other Anonacece. 

 linear rows of pores. 3 i^ ^he twining stem of Uvaria argentea Bl. 



^ Internally the bands of liber formed by the these surfaces even come exactly to form triangles 



transverse section of their fibres are nearly rect- placed with their vertices in alternate directions, 



angular and continuous. Towards the outside and fitting into one another all round the stem, 



they become more irregular, and more or less It is those consisting of parenchyma only that 



rounded externally, while they arc often seg- axe placed vertex inwards. 

 mented into two or three parts by little cellular 



