286 
THE LEAF 
Influences Affecting the Growth of Plants. — Experiments 
have already been performed showing that stems and 
leaves tend to turn towards the light (page 230), and that 
roots normally grow downward (page 230). The stimulus 
that causes roots to grow downward is gravity, and the 
response to this stimulus is geotropism (Greek, ge, earth; 
tropos , a turn). Response to the stimulus of light is known 
as phototropism (Greek, phos, light; tropos, a turn), while 
that of the sun is known as heliotropism (Greek, helios, sun; 
tropos, a turn). When roots turn aside to avoid an obstacle 
and when tendrils or climbing stems clasp a support, they 
are acting in response to the stimulus of contact, the root 
acting negatively and the tendril and stem positively. This 
response is known as thigmotropism (Greek , thigmos, touch; 
tropos, a turn). In general, plants or parts of them show 
positive thigmotropism towards stimuli that are helpful 
and negative thigmotropism towards those that are harmful. 
The roots of the elm and the poplar trees which force their 
way into sewer pipes through a joint and then fill it with roots 
are acting in response to the stimulus of the presence of 
water. This is known as hydrotropism (Greek, hydros, 
water; tropos, a turn). 
LABORATORY STUDY OF LEAVES 
Draw a leaf of geranium (or other plant). Label (1) blade, 
(2) petiole, (3) stipules, if any. Notice the arrangement of the veins 
as you look through it. 
Remove the leaves from an onion. Note the thickened bases and 
how they are attached to the short stem. 
Stand a stalk of celery in water tinted with red ink. Cut across 
it after two hours and observe the position of the vascular bundles. 
Trace them into the leaf. 
Pull off leaves of dock and plantain and observe the tough vascular 
bundles. 
Study a bit of epidermis with a microscope. Draw. Label stomata, 
epidermal cells, guard cells (2 around each stoma)/ Study a cross 
section. Label cuticle, outermost layer, epidermis (upper and under 
