190 



OEGANOGBAPHY. 



scribe these various modifications tinder two heads. 1st, Those 

 kinds of Indefinite Inflorescence with an Elongated Primary 

 axis, and 2nd, Those with a Shortened or Dilated Primary axis. 



1. Kinds of Indefinite Inflorescence with an Elongated Pri- 

 mary Axis. — These are as follows : — 



a. The Spike. — This is a kind of inflorescence in which the 

 primary elongated axis simply hears sessile flowers, or flowers 

 in which the pedicels are very 



Fig. 387. 



Fig. 388. 



short, so as not to be clearly dis- 

 tinguishable. Examples of it 

 may be seen in the Eib-grass 

 {Plantago) {fig. 387), and Ver- 

 vain {Verbena officinalis) {fig. 

 388). In this kind of inflores- 

 cence it will be observed that 

 the flowers at the lower part of 

 the spike are in fruit {fig. 388), 

 while those near the middle are 

 in full flower, and those at the 

 top are still undeveloped. The 

 flowers here therefore open flrst 

 at the base, and last at the 

 apex. Such a mode of opening 

 is called centripetal. This mode 

 of expansion is universal in the 

 different kinds of indeflnite in- 

 florescence, which in all cases 

 open from the base to the apex 

 if the axis is elongated {fig. 388), 

 or from the circumference to- 

 wards the centre if it is de- 

 pressed or dilated {fig. 402). 

 This centripetal order of ex- 

 pansion necessarily arises from 

 the mode of development of 

 such kinds of inflorescence ; 

 thus, the flower-buds situated 

 at the base of an elongated axis 

 are those that are first formed 

 and which are consequently the 

 oldest ; but as the axis elongates 

 upwards, it is continually pro- 

 ducing other flower-buds, the age of which continues to decrease 

 as we approach the growing point or apex ; and as flower-buds 

 are necessarily most developed in the order of their age, it 

 follows that those at the base will open first, and that the order 

 of expansion will proceed gradually upwards towards the apex, 

 or centripetally. In the same way the flower-buds situated at 



Fig. 387. Spike of a species of Rib- 

 grass (Plantago).— Fig. 388. Spike 

 of Vervain (Verbena). The flowers 

 at tbe base may be seen to have 

 already passed into the state of 

 fruit, whilst those at the apex are 

 still unexpanded. 



