56 ON THE PLANT-CELL. 



bundles from the thin-walled short parenchyma around. Such arc 

 monocotyledonous vascular bundles. 



2. Indefinite or unclosed bundles. In these the cambium does 

 not cease to develop, and the vessels to thicken from within outwards, 

 till the organ, or the plant to which it belongs, ceases to live. 

 Such are dicotyledonous vascular bundles, arid they may again 

 be distinguished into, 



. Primary vascular Bundles, those which are produced during 

 the first period of vegetation, or first year. In the inner half it 

 consists of the same parts as the closed bundles, only that the ves- 

 sels are more numerous, and not so regularly arranged ; the outer 

 half is composed of cambium-cells, which are distinct laterally and 

 in front, but pass quickly into the surrounding parenchyma. 



/3. The Wood. After the completion of the first period of vege- 

 tation, the parts of a plant generally cease to increase in length ; 

 but as the new cambium-cells must, nevertheless, extend to a certain 

 length, they necessarily interpenetrate amongst each other by 

 pointed extremities. Thus originates, in the place of parenchyma, 

 a peculiar tissue which is called proscnchyma. A part of these 

 retain their narrow lengthened form, pointed above and below 

 (wood-cells, woody fibres), but between them open individual per- 

 pendicular rows of cells, often very strongly marked, which become 

 converted into vessels. The only exceptions to this are the Coniferce, 

 Cycadacece, and some others, where all the wood-cells are tolerably 

 uniform. The portion of wood that is formed first in every year 

 is composed of broad thin-walled cells, and contains more vessels 

 than later-formed wood, which consists of smaller vessels, and the 

 cells are always narrow and thick-walled. In this way the differ- 

 ence between the growth of the earlier and later periods of the 

 year may be detected by the naked eye. It is from this cause 

 that the trunk of a tree displays, on a transverse section, as many 

 concentric rings as the tree is years old; and these are called 

 annual rings. 



The cellular tissue existing between the vascular bundles and 

 their developing masses, and which ordinarily appear extended from 

 within outw.ards, are called medullary rays. Such extensions of the 

 cellular tissue are large when they reach from the pith to the bark, 

 and small when they begin or end in the wood. 



The Cambium. When the growing parts of plants which form 

 and develope buds are examined, there will be found always present a 

 tissue which is only difficult of rcognition in its individuality. The 

 cells of this tissue, distended with assimilated mucilaginous granular 

 matter, contain young cells, cytoblasts, and often also superfluous nutri- 

 tionary matter, such as starch ; and are pressed by small narrow delicate 

 cells, so that it is very difficult in this tissue to distinguish its component 

 parts. Gradually, separate masses of cells, with a distinct and definite 

 outline, appear in this chaos, and they cease to partake of the process of 

 growth going on. At first the epidermis is separated, then the vascular 



