846 MECHANICS OF GROWTH. 



Jllium Porrum and of Allium Cepa the growth is basal. The position and the form 

 of the geotropic curvature is therefore different in the two cases. 



The length of the growing region, when fully developed parts already exist, is 

 greatest at a certain time, after which it diminishes as the stem gradually approaches 

 its complete development, and finally disappears. The following measurements were 

 made whilst growth was still active and the growing region of considerable extent. 

 The length of the growing region behind the bud was in 



Fritillaria imperialis 7-9 Ctm. \ 



Allium Porrum about 



Allium Cepa 



Allium atropurpureum 



Cephalaria procera 



Polygonum Sieholdi 



Asparagus asper 



Valeriana Phu 



Dipsacus Fullonum 



The measurement of portions of equal length (of one or of five ctm.) shows that 

 the growth of each such portion is greater the more distant the portion measured is 

 from the terminal bud, or, in the case of basal growth, from the base. At a certain 

 distance from the apex (or the base) the maximum occurs; beyond this distance the 

 growth of each portion diminishes until it altogether ceases at the limit of the growing 

 region. The form of the geotropic curvature and its modifications essentially depend 

 upon these conditions. In these particulars stems which consist of numerous internodes 

 but without well-defined nodes {Asparagus) resemble long single internodes, such as 

 the scapes of the various species of Allium. If however the stem is distinctly articulate, 

 a curve of fractional growth may be obtained from each internode, which rises as we 

 pass from its lower end until a maximum is reached, and then sinks as we pass towards 

 its upper end. The node itself ceases to grow in length at an early period. As the 

 result of this, the geotropic curvature of the whole stem is interrupted at the nodes, 

 and the quickly-growing central portions of the internodes describe sharper curves. 

 With the exception of this peculiarity, an articulate stem behaves generally in the 

 manner above described with reference to a long scape consisting of a single internode. 

 Finally, it is to be noted that each transverse zone of a growing stem grows at first 

 slowly, then more rapidly until a maximum rapidity is attained, and then more slowly 

 until growth ceases altogether. This also determines the form of the geotropic 

 curvature. 



Those portions of a stem which have ceased to grow and which are incapable of 

 renewed growth in consequence of a change of position (see what is said below about 

 the nodes of the haulms of Grasses) will not assume an erect position when placed 

 horizontally or obliquely. Only those portions of a stem laid horizontally or obliquely 

 take part in the assumption of an erect position which are growing (as in the down- 

 ward curvature of the root), and this in proportion to the phase of their growth, their 

 thickness, rigidity, etc. This curvature is a consequence of a modification of the growth 

 in length of the stem produced by its abnormal position, of such a nature that the 

 growth of the under side is more rapid and that of the upper side less rapid than that 

 of the stem in the erect position. In quickly-growing parts the upper side evidently 

 increases in length when the geotropic curvature is taking place, but in older more 

 slowly growing parts the length of the upper side does not increase, and it may even 

 become a little shorter if the curvature is very sharp, whilst the lower side elongates 

 considerably. These statements can be easily verified by direct measurement of thick 

 firm stems before and after the curvature has taken place. It is obvious that the 

 convex side of a curved stem must be longer than the concave, but the question as 

 to whether or not the growth of the concave side is slower and that of the convex 



