208 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



(a) the tendrils in the Pea, in Gloriosa, in Smilax and iu 

 Cucumber, growths that have to assist the stem in climbing, 



(h) the thorns in Acacia, or in Jujube, these growths being 

 metamorphosed stipules, and serving as instruments of defence 

 to the plant against the attack of herbivorous animals, 



[c) the bladders of Utricularia, which are traps by means 

 of which the plant catclies small animals on which it feeds. 



B. The Work done by the Leaf. 



1. Transpiration or Evaporation of Water. 



{a) The Fact that Plants evaporate Water can be proved by an 

 experiment. Place some fresh twigs of a plant under a bell-jar 

 in the sunshine. After a short time we shall find a deposit of 

 moisture on the inner side of the glass. Another bell-jar with no 



plants under it, similarly placed 

 in the sunshine, has no such 

 deposit of moisture. From this 

 we draw the conclusion that 

 plants transpire or evaporate 

 water in the form of water- 

 vapour. 



{b) How can this Transpir- 

 ation take place? — A careful 

 examination of the surface of 

 leaves shows that there are 

 many tiny openings which, look- 

 ed at through a microscope 

 (fig. 193), appear like little 

 mouths, and are therefore called 

 stoniata (from the Greek stoma, 

 mouth) . Some leaves have them 

 on l)oth sides, but most only on the under side. (The Lotus 

 plant cannot but have them only on the u])[)er side. WhyV 

 Compare page 3.) These openings lead to hollow spaces, called 

 air -cl I ambers, in the interior of the leaves (see fig. 181 on page 198). 

 Tiie stomata are tlie "gates" of these cavities, througli wliicli a 



Fig. 193 — Part of the surface of 

 a leaf. S. Stoma (2i)0 times enlarged). 



