234 STRUCTURE AND VITAL PROCESSES OF PLANTS 



3. Direction of Growth influenced by Gravity and Light. 



{(i) By fjravity. — Plant a few germinating seeds of Horse 

 Gram in various positions, some Avitli their liypocotyl up, others 

 with their hypocotyl down, and others again with their hypocotyl 

 horizontal: the stems of all these plants will invariably rise 

 vertically up when they grow. Observe a cocoanut tree thrown 

 over hy a storm: its shoot will raise its liead straight up, after 

 a time. Plant cuttings of Croton in a slanting position: the new 

 shoots will grow upright Stems, as a rule, grow erect, even on 

 sloping ground. This is due to gravity. Gravity causes such 

 stems that are disturbed in their natural vertical growth to grow 

 stronger on their lower side till they regain their original di- 

 rection of growth. 



The main root grows in the opposite direction of the stem, 

 following tlie law of gravity similarly. The main root is, there- 

 fore, said to be geotropic, and stems are negatively geotropk. 



The branches of a tree cannot grow straight up likewise; 

 owing mainly to the influence of light, they grow obliquely or 

 even horizontally. 



(hj Light. — Stems planted near a house do not grow straight 

 up, but incline away from the house towards the light. Light 

 causes the shaded part of a shoot to lengthen, more than the part 

 exposed to it, and thus the direction of growth is changed. A 

 plant growing free in the open air is exposed to the liglit all 

 round in the same degree, and hence all parts of the stem grow 

 e(jually, and the stem becomes erect. 



lioots, on the other hand, show the opposite tendency. In the 

 hanging roots of the banyan tree, for instance, it may be noticed 

 that they bend towards the dark side, i. e., away from the light. 

 Stems are, tlierefore, called Jieliolropic, and roots negatively 

 heliotropic. 



4. Habits of Stems. 



Most stems are erect (Suntiower, Mangoj. P^it some are weak 

 and lie on the ground raising their heads gradually: ascendent 

 (Desmodium tri([uetrum); or so weak as to lie Hat on the ground: 



