GROWTH PHENOMENA CONTROLLED BY INTERNAL CONDITIONS 253 



§3. Tissue Strains." — Each plant organ consists of many kinds of tissues, 

 and the different sorts of cells do not divide and enlarge at a uniform rate. It 

 thus follows that opposing forces, or stresses, develop between the tissues, one 

 tissue pressing against another while the latter, in its turn, also tends to enlarge 

 and press against the former. Thus result what are called tissue strains, 

 which increase the rigidity of plant organs. In every plant some organs are in 

 a state of strain by traction (i. e., they are stretched), while others are under 

 pressure (i. e., they are compressed). Strains may occur either longitudinally 

 or transversely. Longitudinal strains may be easily demonstrated. Two 

 longitudinal cuts are made, perpendicular to each other, through the center of 

 a dicotyledonous stem or the flower stalk of one of the Liliaceae or of Taraxacum 

 (dandelion), which is still elongating. The four strips of stem thus formed bend 

 outward, the originally outer surface becoming concave. From this it follows 

 that the epidermis and cortex are stretched in the uncut stem, while the pith is 

 compressed. Splitting the stem allows the pith to expand and the cortex to 

 contract. Each concentric layer of tissue in an internode that is elongating, 

 is stretched with respect to-the next layer within and compressed with respect 

 to the next external layer. If the strips just mentioned are placed in water the 

 bending becomes more pronounced, and frequently results in coiling. 



Transverse strains may be seen best in old stems of dicotyledonous plants. 

 These strains are produced by the occurrence of more rapid enlargement in the 

 wood than in the bark, so that the latter is stretched and the former compressed. 

 If a girdling band of bark is removed from such a stem (willow, for example), 

 and if it is then returned to its original position, the two ends fail to meet, be- 

 cause of the fact that the band contracted as it was removed. 



Summary 



1. The Grand Period of Growth. — The enlargement of a plant, organ, or tissue 

 begins at a slow rate, and the rate gradually increases to a maximum, after which it 

 progressively decreases until enlargement ceases altogether. The time period corre- 

 sponding to this march of the elongation rate is called the grand period of enlargement. 

 Each cell has its grand period. The influence of external conditions may lengthen 

 or shorten the grand period or may alter the maximum rate of enlargement, but the 

 general march of the rate is essentially controlled by internal conditions. For ex- 

 ample, the terminal internode of a seedling bean elongated 1.2 mm. on the first day, 14 

 mm. on the seventh day, and 2 mm. on the tenth day. 



a The word strain is here used in its mechanical sense, meaning any sort of deformation, 

 whether of tension (enlargement), of compression or of shearing (changes of shapes without any 

 change of volume). Many writers of English still use tension where strain is here employed, 

 being thus led to the awkward teutonicism by which compression is called negative tension. It 

 may be remarked that the force that tends to produce any kind of strain (whether actual defor- 

 mation occurs or not) is to be called a stress, so that there are three kinds of stress correspond- 

 ing to the three kinds of strain above mentioned. In this connection, see Ewart's remarks in 

 v. 2, p. 62, footnote 1, of: Pfeffer, W., The physiology of plants. Translated by A. J. Ewart, 

 Oxford, 1903. — Ed. 



