88 SENSITIVENESS OF 'STEM 



quently they adjust themselves to the best advantage (see page 



29). 



As a rule the irritability or sensitiveness of the stem is not so 

 localized as in the case of the root. It is evident that it is an 

 advantage to have the entire growing surface of the stem sensi- 

 tive to all those stimuli that influence its growth, because by this 

 means it will be so influenced as to bring the leaves and branches 

 into proper relations with the surroundings. In some plants to 

 be sure the sensitiveness to stimuli is quite as localized as in 

 the root ; as, for example, the tip of the cotyledons of many grasses 

 which have an appreciation of light almost the equal of our eyes. 

 These cotyledons will turn toward a light too feeble to enable us 

 to read the time of day on a watch. The significant fact to be 

 noted here is that this keenness and localization directs the plant 

 into a suitable place as soon as it emerges from the soil. Grass 

 plants grow in thick colonies and therefore the stems come into 

 sharp competition. Consequently it is a necessity that the young 

 shoot on emerging from the soil should be able to appreciate the 

 feeblest light and so be able to direct its growth through the 

 smallest openings. 



The reactions of the stem to various stimuli, such as light, 

 gravity, etc., are quite as purposive as those noted in the root. 

 Lateral light, as in the case of plants growing in windows, acts 

 as a stimulus and causes a more considerable growth of the cells 

 on the darker sides of the stem. This results in bending the 

 stems towards the light and consequently in the exposure of the 

 broad blades of the leaves to the light. So, also, when erect stems 

 are placed in a horizontal position the stimulus of gravity awakens 

 .a more active growth on the underside of the stem, thus bending 

 the tip into an erect position for securing light. Not all stems 

 and, indeed, not all parts of the same plant body are similarly 

 influenced by these forces. Many plants tend to arrange their 

 stems at right angles to the action of gravity and light, as in the 

 case of creeping and underground stems. So also the lateral 

 branches are inclined at various angles to the stem. These ad- 

 justments are not to be looked upon solely as the direct response 

 to one or more stimuli but as the resultant of numerous interac- 



