104 CLASS-BOOK OF BOTANY. 
CHAPTER VII. 
MoDIFICATIONS OF ORGANS, ETC., FOR SPECIAL PurRpPosss. 
In the preceding chapters incidental reference has been made 
and attention drawn to various modes in which plants have 
their stems, leaves, stipules, and other organs altered to serve 
different purposes. Various modes of, and devices for ge- 
curing, cross-fertilisation of the flowers and distribution of 
their seeds have been mentioned. It will, however, be useful 
here to summarise a few of the principal facts learned, and to 
supplement them by mention of other interesting and remark- 
able modifications. 
Leaves have for their principal function the process of 
conyersion of carbon-dioxide (carbonic-acid gas) and water into 
starch, with evolution of oxygen gas, and for this purpose 
their usual construction is that of a framework of ribs and 
veins on which their softer green tissue is spread. The chief 
function of stems is to support the leaves and the reproduc- 
tive organs. But we have already seen that many stems 
have lost this function, or have modified it so as to gain the 
same end with a minimum development of supporting-tissue. 
This is frequently attained by the habit of twining com- 
mon to so many plants (e.g., French Beans), a habit which is 
sometimes aided by roughnesses on the stems (Hop, &c.). In 
other cases we find (1) recurved prickles developed on the 
mi ‘ 
Fig. 208. Tendril-bearing leaf of Sweet Pea. 
