156 



THE LIFE OF THE PLANT 



but communicate with each other by means of pores, 

 larger or smaller as the case may be. One form of such 

 tubes is very curious : the transverse walls of the com- 

 ponent cells are perforated 

 with minute pores and form 

 a sort of sieve, and the cells 

 themselves are called sieve- 

 tubes. One such cell fused 

 with two others into a single 

 tube is represented in fig. 43, 

 n. The contents of these 

 cells can communicate 

 through their pores. Very 

 minute grains of starch have 

 been observed in these pores, 

 and we shall soon see how 

 important is their physio- 

 logical significance. As well 

 asthevessels and sieve-tubes, 

 which are long and straight 

 tubes, we come across others which branch and inter- 

 weave and form a whole complicated network of com- 

 municating canals. Such tubes generally contain white 

 or sometimes yellow sap ; hence their name of latex 

 tubes. Plants containing latex are known more or less 

 to everybody ; e.g. the dandelion and the poppy which 

 exude a white sap when slightly wounded ; the blood- 

 wort, probably known to us all since our childhood, 

 from the injured stems and leaf veins of which a yellow 

 sap escapes ; lastly, the common Ficus, which is grown 

 indoors with us, and which, like some other tropical 

 plants, secretes an abundant latex, known under the 

 name of india-rubber when it is dried. These different 

 kinds of latex are contained in a complicated system of 

 branching and interwoven tubes to be found all over the 

 plant, but especially in its rind and leaves. 



All these elementary organs of the plant can be divided 



Fig. 43. 



