172 STEMS 



ment of the plant from year to year, no increase in the 

 amount of foliage display, no gradual spread of branches. 

 In temperate regions this lack of cambium seems to compel 

 the annual habit, at least so far as the aerial parts are con- 

 cerned ; very few stems with closed bundles live through 

 severe winters. Evidently the annual habit and the lack 

 of facilities for secondary growth have something to do 

 with each other. 



The stems of some tropical and semi-tropical plants 

 which have scattered bundles, especially those of palms 

 and yuccas, do increase in diameter from year to year. 

 This is due, however, to the activity of a cortical cambium. 

 There is no cambium in the vascular bundles. 



Many familiar plants besides corn have the scattered 

 arrangement of vascular bundles. One of the greatest of 

 the families 1 of plants is the grass family. To it belong 

 all the cereals except buckwheat, as well as all the kinds of 

 grass. Another large plant family is the lily family, of which 

 asparagus and onion and tulip are familiar examples. All 

 the members of the grass and of the lily families, and many 

 other plants which are related to them, have this scattered 

 arrangement of their vascular bundles. 



You will recall that the tall grass stems you have pulled 

 in summer are hollow, and a stalk of wheat has already 

 been given as an example of a hollow stem. Evidently 

 hollowness in stems is not limited to those with the cylin- 

 drical arrangement ; in fact, it is more common among 

 stems with the scattered arrangement. As hollowness 



1 Frequently from this time on you will meet the word family as applied to 

 a group of more or less similar plants. Botanists have arranged plants into 

 groups which they call families for the reason that such groups are deter- 

 mined by what are believed to be natural relationships or kinships. 



