554 SUMMARY OF CURRENT KESEARCHES RELATING TO 



pendicular to the leaf -surface, between the secondary bundles and the 

 assiniilatory zone. The regular arrangement allows free access of light 

 to the aqueous tissues, and is only found where tannin is present ; if 

 the cells contain simple mucilage, they are much more irregular. This 

 fact appears to support the popular view that the chief function of 

 aqueous tissue is the improved illumination of the interior of the leaf. 



Structure and Development. 

 Vegetative and Reproductive. 



Cell-arrangement.* — F. Neeff has studied the changes in the 

 arrangement of the cells resulting from decapitation of the main shoot. 

 The following species were studied — Tilia americana, T. platyphyllos, 

 JEsadus Hippocastanum, Acer platanoides, Salix babylonka, Picea excelsa, 

 Populus pyramidalis, Ricinus communis, and Phytolacca icosandra. 

 Investigations show that a series of changes takes place in the cambium 

 of the nearest lateral branch, in the zone adjoining its attachment. The 

 cambium-cells divide by short cross-walls to form wood and bast 

 parenchyma, but the new growth is in the direction of the lateral branch. 

 The short cambial elements give rise to large bundles, which run more 

 or less transversely ; fusion then takes place, usually by the absorption 

 of longitudinal walls, and thus it is possible for the wood of the main 

 stem to supply water to the lateral branch. Analogous changes take 

 place in the sieve-tubes. Subsequent cambial growth results in the 

 formation of wood and bast, which from the first is in the new direc- 

 tion, and in which the normal fusion by the absorption of transverse walls 

 is again resumed. Similar changes result from wounding of the main 

 axis ; while if both decapitation and wounding occur in close proximity, 

 the formation of new tissues is accelerated. In the case of wounded 

 tissues, new vascular strands are formed transversely, so as to unite the 

 normal strands which run longitudinally on each side of the wound. In 

 the case of completely inverted plants the polarity of the tissues is like- 

 wise inverted, so that the cambium gives rise to root-structure in the 

 former apical region, and vice-versa. This change in the direction of 

 the tissue formation is accompanied by a sliding growth of the individual 

 cells, during which the protoplasmic connexions are severed, and each 

 cell has a relative independence of movement ; as new tissues are formed, 

 however, this independence disappears, and new protoplasmic connexions 

 are formed with adjacent cells. The author agrees with Vochting as to 

 the less definite polarity of young parenchyma-cells, and upon this fact 

 bases a new theory, according to which the cambium in the neighbour- 

 hood of a wound functions as a growing-point, which gives off new cells 

 in all directions, capable of responding to a new polarity ; in this way 

 the changes described above are perfectly comprehensible. In conclusion, 

 it is shown that, since neither interruption of the flow of water nor of food- 

 materials can alone bring about change of direction in the formation 

 of new tissues, this change must be due to some property inherent to the 

 contents of the cambium-cells. 



* Zeitschr. Bot., xvi. (1914) pp. 465-547 (2 pis. and 32 figs). 



