STEMS 33 



strawberry plant. They are attached to the old plant for 

 but one season. Runners may branch. 



6. Lianas. — ^A liana is a climbing stem, gaining mechanical 

 support only from another plant. Common hanas are the 

 grape, Virginia creeper, hop, Japan ivy {Psedra tricuspidata) 

 and morning glory. The stems of lianas are slender, long, 

 and have insufficient' strengthening tissue to hold them 

 perfectly erect. Hop stems always wind about the support 

 clockwise (Fig. 102). Such a twiner is destrorse. The twin- 

 ing stem of Virginia creeper bears fleshy, yellowish air roots 

 which may aid the plant in adhering to its support. Of 

 greater value to the Virginia creeper plant, in this regard, 

 however, are the highly specialized branches — tendrils. In 

 this case, a tendril ends in a knob which flattens out, when 

 it comes into contact with a surface, and adheres to that 

 surface by a mucilaginous disk-shaped structure. 



7. Spines.- — Some spines are reduced stem structures, as 

 is the case in the honey-locust, hawthorn, wild crab, etc. 

 Many small spines, such as are found in gooseberries, cacti, 

 and roses, for example, are outgrowths of the stem. It 

 seems that spines are induced by an excessive loss of water 

 from the plant, and a low absorption rate, such as occur 

 under desert and semi-desert conditions. 



STRUCTURE OF STEMS 



The Young Dicot Stem.— Let us cut a middle lengthwise 

 section of a young dicot stem (Fig. 15). This section will 

 cut the growing point (bud) of the stem, and the older parts 

 back of the growing point. We see that the stem becomes 

 progressively older farther and farther back from the tip. 

 The cells at the growing point make up a tissue known as 

 meristem tissue (undifferentiated tissue).. Although they 

 3 



