MOVEMENTS OF GROWTH. 397 



to the protoplasm. One may imagine that the cell membranes 

 acted upon by the stimulated protoplasm undergo qualitative 

 changes, which result in retardation of their growth. 



If we examine the epidermal cells of the long leaves of many 

 monocotyledons, we find that they are much elongated. This is 

 due essentially to the fact that the epidermal cells of these organs 

 are stretched by the tissue tensions mainly in a longitudinal 

 direction. If we examine a fragment of the epidermis from a leaf 

 of Syringa or some other dicotyledon, it will be seen that the 

 epidermal cells are in the form of polygonal plates, which is 

 certainly related to the fact that the surface development of the 

 leaves takes place in almost the same manner in two directions. 



We prepare transverse sections of a twig of Tilia parvifolia, 

 about 5 mm. in thickness. The structure which reveals itself on 

 microscopic examination has already been described in 42. We 

 are here only concerned really with the wood of the A r ascular 

 bundles. We see that several annual rings are present. The 

 spring wood passes quite gradually into the autumn wood of the 

 same ring, while the spring wood of the next ring is quite sharply 

 separated from the autumn wood of the previous year. The 

 spring wood is specially characterised by the presence of large 

 vessels. Later on these more and more completely disappear. 

 The autumn wood consists entirely of elements with narrow 

 lumen. 



Sachs long ago pointed out a connection between the difference 

 in structure of the spring and autumn wood on the one hand, and 

 the varying intensity of the transverse tension during a vegetative 

 period on the other. In the spring the cortex is clearly less 

 stretched than at a later period, when the formation of wood has 

 made further progress. In the spring, therefore, the pressure to 

 which the wood is subjected is less than at a later period, and in 

 these circumstances is to be sought the chief explanation of the 

 fact that the wood elements formed from the cambium cells at 

 the commencement of the vegetative period have wide lumina, 

 whereas later, especially towards the autumn, only elements with 

 small lumen are formed. 



We will now see how plant structures growing in thickness are 

 affected when artificially subjected to increase and diminution of 

 pressure. We may employ two to three-year-old branches of various 

 shrubs or trees. Increase of pressure we obtain by winding a 

 piece of string, not too thick, for a few centimetres round the 



