In this physics PHD thesis applied to vegetal physiology we focus on two phenomenons dealing with plants. The capillary ascension of liquids in corners and lodging of cereals and trees. When plants are under high hydraulic stress, we observe gas bubbles appearing in their sap, this is gaseous embolism of plants. Hence, there are situations with sap/air interfaces in plants. And we observed on anatomic views of plant vessels geometries with corners. Those geometries and the fact there are liquid/vapor interfaces enable the capillary ascension of sap. This could be a mechanism for embolism repair in filling back emptied vessels or in assuring a safety flux of sap while new vessels -full of sap- form in the plant. We have characterised the dynamics of capillary ascension in a various panel of corner geometries and applied it to the ascension of liquids in stems emptied from their sap. Still in vegetal domain, we study lodging, that is the falling of plants. Here we focus on the falling due to wind and by the mechanism of stem breaking. Lodging by wind is very frequent for cereals, it causes a loss of 30% of the harvest, it is a great deal for farmers. During great storms, forests are also damaged, we have had recently in France the Lothar and Martin storm in 1999 and Klaus storm in 2009. Comparing the speeds of wind during the storms and the percentage of trees damaged shows that above a critical wind speed almost every trees fall, that speed is 150 km/h. Looking at curved geometry of cereals and brittleness of trees we could deduce criterions of lodging and especially predict wind lodging speeds
