174 A TEXTBOOK OF GENERAL BOTANY 
storage are underground ones, there are exceptions. Some palms 
have greatly thickened stems in which large quantities of starch 
are stored during a series of years. This food is finally used up 
in the formation of flowers and fruits, after which the stem dies. 
Commercial sago is made from the starch stored in the trunks 
of the sago palm. 
Some stems are especially thickened and modified for the storage 
of water. This is particularly true of the cacti (Figs. 512, 514). 
| In the Malayan 
/ region there are cer- 
| tain curious epiphytic 
plants that may be 
Zp mentioned in this con- 
=p nection. The basal 
ZL L LLCO, ortion of the stem 
ZS? 
7 is greatly enlarged 
and consists mostly 
of water-storing tissue 
(Figs. 176, 1772s 
this basal portion are 
conspicuous labyrin- 
thine cavities which 
are connected with the external atmosphere by means of small 
openings. The cavities are inhabited by ants. Various functions 
have been assigned to these cavities by different botanists, while 
others think that they have no particular function. Some regard 
them as devices for aération, others consider that the ants which 
inhabit them benefit the plant by leaving débris from which the 
plant absorbs nutrient material, while still other observers believe 
that the ants serve as a means of defense for the plant. 
Protection. ‘he spines of many plants are modified branches 
(Fig. 178). In some cases these spines protect the plants to some 
extent from being eaten by browsing animals. 




Fig.178. Branches of lime (Citrus aurantium) 
modified as spines. (x +) 
