88 PLANT PHYSIOLOGY 



dinally, the two halves become convex to each other ; thus, the median region 

 of the cortex shows the greatest tendency to extension. Further, in the root 

 also there exists a very obvious tissue tension at a certain distance from the 

 growing point. If a longitudinal slice be taken from the full-grown region, one 

 notes an effort to expand both in the central and in the peripheral regions ; the 

 cambial region is in a state of negative tension. If the lamella be cut into 

 four pieces the curvatures correspond to those just recorded. The effect of 

 the effort to contract on the part of the cambial layer has been already 

 described. 



In addition to longitudinal tensions one has to note also transverse tensions 

 which exhibit themselves especially in such parts of the plants as develop 

 secondary thickening. If one removes a ring of cortex down to the cambium 

 and then endeavours once more to replace it in its old situation, it is found to be 

 too short ; it has, in fact, contracted. 



The amount of tissue tension is not constant ; on the contrary, it shows 

 a striking periodicity which has been especially studied by KRAUS (1881, 1895). 

 He found the maximum to occur early in the morning, and the minimum after 

 midday. Since this periodicity is probably to be referred to the influence of 

 external factors, we will not discuss it further at the present moment. In 

 conclusion we may remark that tissue tensions contribute materially to the 

 rigidity of plant organs ; and this, too, for the same reasons that osmotic 

 tension of the cell-wall induces an increase in rigidity in the individual cell. 

 It may be assumed that herein lies the significance of tissue tension in the plant 

 as a whole. Tissue tension has been studied in the past with great care, for it 

 was expected that conclusions as to various physiological phenomena might 

 be obtained from such studies. These expectations have not, however, been 

 completely realized, and hence we shall not devote any more space to their 

 consideration, but rather return to our discussion of the differentiation of cells 

 at the growing point. 



In addition to the form of the cells, the nature of their membranes is of impor- 

 tance. It is impossible to go into the question of the differences, both chemical 

 and physiological, which show themselves in cell-walls during their formation, 

 but we must study to a certain extent the sculpture of the cell-wall. It is very 

 rarely the case that the cell-wall is thickened uniformly. We will not enumer- 

 ate here the forms which this unequal thickening takes on, but pay attention 

 more especially to the pits, that is to say, the regions which remain thinner 

 than the regions immediately surrounding them. The most remarkable point 

 about these pits is, undoubtedly, the fact that, generally speaking, in two 

 neighbouring cells with one cell-wall, the pits on either side correspondingly 

 placed, the closing membrane is common to both. If this common wall always 

 arose by cell division, correspondence of the pits would not appear to be so very 

 remarkable ; for the pits would be formed at the same time that the wall was 

 formed, and would have a certain relationship to the spindle threads which 

 stretch from one pole of the dividing nucleus to the other. Corresponding pits, 

 however, occur in cell-walls which, for the first time, come into contact with 

 each other later in their history, as, for example, between latex tubes and their 

 surroundings (and these latex tubes push their way into these environments by 

 sliding growth) ; further, in the case of tyloses (STRASBURGER, 1901), how is it 

 possible for a tylose to know where its neighbouring cell has formed a pit ? In 

 all probability the formation of pits comes about in another way, namely, by 

 piercing of the young cell-wall by a whole series of exceedingly fine pores through 

 which the protoplasts of neighbouring cells come into communication with 

 each other. These protoplasmic bridges, which have been shown to occur with 

 great frequency in higher and in lower plants, and also in the tissues of animals 

 (compare especially KUHLA, 1900 ; STRASBURGER, 1891 ; A. MEYER, 1902), are 



