257 

 5. Vascular-b'gndle Tissue . 



T]»e same plasticity exists in vascular bundle tissue, as 

 m ground tissue. 



To, arresting^ influences of very different kinds, the plant 

 reacts especially ty the production of abnormally narrow ducts. 

 J-x IS known that under the action of continued and powerful dis- 

 turbance, the normal composition nf the xylem is lost, the libri- 

 torm fibres disappear, ultimately the ducts also, resulting in a 

 severe attack, in a completely homogeneous, parenchymatic tis- 

 sue I callus tissue -gall wood). The phloem experiences a simi- 

 lar hypoplasia. ^ 



„. . favorable influences call forth abnormally large cells. We 

 imd that the b^rk grows out to hyperhydric tissues through the 

 nypertrophy of its different elements, the wood-parenchyma cells 

 form tyloses etc. or very extensive excrescences in the form of 

 callus, wound-wood or galls are produced. In all essential 

 points, the same holds good for this as for ground tissue. 



Of the different parts of the cambium layer, that formed 

 from the embryonic tissue, the cambium itself, it always capable 

 of accomplishing the most. Yet even the tissue of the phloem, 

 as shown above, can grow out into extensive callus roajS or fur- 

 nish large galls, rich in cells. The smallest part in the pro- 

 duction of abnormal tissues is performed by the xylem, of v®,ich 

 in fact the elements are mostly dead or lignified. Attention has 

 already been called to tyloses. All other phenomena, known in 

 the elements of the woody -part, play only a subordinate role. 



If, a^ter these brief discussions, we condider once more 

 . » t"^^ different cell tissue forms arising during abnormal growth, 

 1299) it, may be affirmed, that the derivatives of very different kinds 

 of tissues can assume very diverse forms. In the ^orsiBtioa »f 

 galls true epidermis, bearing trichome'S can be produced from the 

 mesophyll (compare for example, fig. 87;, nighty sclereid con- 

 plexas can be derived fron the delicately walled nesophyll (fig. 

 90) , true epidemis and typical ground tissue with stone cells 

 can develop from iKascular bundle tissue etc. in the formation of 

 galls* - In the formation of callus, we find the production of 

 derivatives of the pith, the secondary bark etc. and of tracheids 

 etc* In short, fron each tissue all can be formed, the course of 

 their development is deteruined by the sun of all the factors 

 acting on then. V/e find callus "developing fron the pith of 

 wounded sprouts and the production in it of the sane tracheal ele8 

 nents which under normal conditions are net with only in the xyler: 

 If in the interior of the nomally developed shoot a cylinder of 

 ground tissue is deve lofted, instead of i^ascular bundle tissue 

 containing tracheids, it is not due to any inherent peculiarities 

 of the cells lying centrally, but is the result of all (internal 

 and external) factors which have been active during the whole 

 developmental period. Further, not only the cells of the derna- 

 toeen can develop "typical" epidemis bearing trichones, but also; 

 as proved by galls,- all other tissue forns. Further, the "nor- 

 mal" maturing of the ground tissue into assinilatory, palisade 

 parenchyma, etc. is not a result of its specific constitution, 

 which v^ould pernit of inly one developnental course, but is con- 

 ditioned by the factors, to the action of v;hich the self-develop- 

 ing ground tissue is subjected under nornal conditions. The fact 

 that we always find the sane tieeues in the sane order, in nor- 

 mally developed organs , proves nothing further than that in 

 then the decisive aotive factors always cone to expression in 

 the sane way. 



