58 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



2. CAUSES OF ARREST. 



We have here to consider two sets of causes which may however 

 come into consideration together : 



(1) The arrest may take place through the influence of other parts 

 of the same plant, that is to say, directly through correlation. 



(2) The arrest may be a consequence of the loss of function of the 

 organ, and the reduction of the organ may be caused either directly by 

 its loss of function, or only indirectly, inasmuch as an arrest of useless 

 organs will benefit the others, at any rate there will be less demand 

 upon the plastic material. The loss of function takes place in many 

 cases by another organ taking over the function in question, for example, 

 the shoot-axis may take on the function of assimilation which commonly 

 is the work of the leaf. 



(1) Arrest through correlation. Numerous examples may be given of 

 this, but as these will be specially dealt with in the chapters treating of the 

 phenomena of correlation a brief reference is all that is necessary here. 



The prothalli of ferns and the protonemata of mosses, with the 

 exception of that in Ephemerum, die off when a young plant has arisen 

 upon them and takes off to itself the plastic material. In the ovary 

 of the oak one only of the six ovules develops, in the case of the lime 

 one only of ten ovules develops, and supplants all the others. The upper 

 flowers of the many-flowered inflorescences in Boragineae, Oenothera, and 

 other like plants, are arrested if the lower flowers set seed. The buds at 

 the base of the annual shoots of most broad-leaved trees are quite as 

 capable of development as the others, but owing to their position they 

 are arrested and remain as ' resting buds,' and only under definite external 

 conditions elongate into shoots. Many other instances occur. 



(2) Arrest due to loss of function. It is not always easy to prove why 

 the loss of function takes place ; that it does occur is very evident in 

 most instances. A number of cases taken from the vegetative region will 

 be found in the chapter upon relationships of symmetry, and I will only 

 refer to a few here. 



In the inflorescence of Lolium, which was referred to above, one 

 of the two glumes which in grasses usually invest the spikelet is 

 suppressed in the lateral spikelet, but the terminal spikelet possesses 

 both of them. This happens in the terminal spikelet because it lies 

 free and requires a protection upon every side, but the lateral ones lie 

 in a depression of the axis of inflorescence which covers them on one side 

 whilst the other side is protected by the lower glume; the upper glume 

 on the side next the axis of inflorescence is here quite superfluous 

 and is consequently suppressed. The bracts of many flowers show similar 

 phenomena. Where the flowers stand closely grouped together their 



