526 PHYSIOLOGY. 



and scalariform vessels, surrounded by parenchyma. The bundles are 

 regularly developed onwards with the growth of the point of the stem, 

 sending off lateral branches of spiral vessels where leaves arise, but 

 undergoing no change after the internode in which it lies is once formed. 



Equisetaceae. In the Equisetacere (p. 417) a ring of isolated fibro- 

 vascular bundles exists in the periphery of the aerial stem, surrounded by 

 liber-cells and parenchyma ; these, again, are closed bundles, and grow 

 only at their points as the stem elongates. The constituent vessels are 

 spiral or annular. The epiderm, and specially its hypoderni, fibres are 

 highly developed. The Equisetaceae are the only plants known whose 

 buds originate, deep in the substance of the stem. 



Filices. In the Ferns (p. 419), where the stem acquires greater di- 

 mensions, we find a number of fibro-vascular bundles standing in an ir- 

 regular circle, surrounding a central cellular axis, and externally sur- 

 rounded by a kind of rind (liber) containing sieve tubes. The bundles do 

 not run straight up the stem, but in waved curves ; and they anastomose 

 laterally and separate again, leaving wide passages of communication 

 between the central parenchyma and the rind (fig. 573). The branches 

 of the bundles going to supply the leaves are given ofi'at the anastomoses 

 of the main bundles ; and the bundles running into the (adventitious) 

 roots arise at similar places. The bundles of the stem have only indirect 

 connexion with those that pass to the leaves, so that in leafless parts of 

 the stem the arrangement is the same as where there are leaves present. 

 These bundles are closed, and therefore the stems never alter in dimensions 

 when once formed. In cases like that of Angiopteris evecta, w^here the 

 stem is reduced to very small proportions, there are, according to Mettenius, 

 three zones of fibro-vascular bundles, one within the other, and connected 

 by intervening net-like bundles. It was formerly supposed that vessels 

 of the scalariform type were the only ones that occurred in Ferns ; but it 

 is now well known that spiral and annular vessels also occur, especially in 

 the younger portions. 



All the above forms of the stem are characterized by having their 

 fibro-vascular bundles when complete destitute of cambium, hence called 

 closed (p. 515). They are developed only at the point. From this 

 circumstance, these higher Cryptogarnia are often called Acrogens, or 

 Acrobrya (point-growers). 



Monocotyledons. The stems of Monocotyledonous plants have a very 

 different organization from the above. The most striking peculiarity, at 

 first sight, is the isolation of the fibro-vascular bundles, which, as a rule, 

 anastomose but slightly in any part of their course through the stem, and 

 are scattered singly in the parenchyma of the stem (fig. 574). Another 

 important circumstance is, that they pass entirely into the leaves at their 

 upper ends (fig. 575, a), while at their lower extremities they approach 

 the surface of the stem and anastomose with their fellows to form a more 

 or less developed fibrous network, separating the rind or cortical paren- 

 chyma from the central fibrous part of the stem. It is from this network 

 that the fibro-vascular axes of the (adventitious) roots are derived (fig. 

 575, 6). 



The stems of Monocotyledons are very generally herbaceous, and thus 

 present very important varieties of form, arising from non-development of 



