56 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



ARREST \ 



The more elaborate the differentiation a plant exhibits the more 

 common is the appearance which we designate as arrest. In Thallophyta 

 it appears relatively seldomer than in the higher plants, and in the latter 

 it is more common in the region of the flower than in the vegetative 

 region, in correspondence with the higher differentiation exhibited there 

 We leave of course out of consideration in this all the cases in which an 

 organ is retarded in its development by unfavourable external influences ; 

 here we have only to do with forms of arrest which are due to inner 

 causes. The following general statements may be made regarding these : 



J. KINDS AND MANNER OF ARREST. 



Arrest is brought about by the primordium of an organ not passing 

 through its complete course of development but remaining stationary 

 at an earlier or later stage. In some cases the primordium of the organ 

 can be proved to have a developmental existence it is aborted. In 

 numerous other cases however the first laying down of the primordium 

 does not take place it is suppressed ; arrest can then only be determined 

 by comparison with other forms. There is however no sharp line between 

 abortion and suppression ; one and the same organ may be sometimes 

 aborted, sometimes suppressed, in the same plant, and this is again only 

 a special illustration of the general rule that organs liable to arrest show 

 great variation in the degree of development at which they arrive. The 

 following are some examples : 



The spikelets of species of Setaria, Pennisetum, and other grasses 

 are surrounded by an envelope of bristles. The history of development 

 shows that these bristles are unquestionably branches of the inflorescence 

 on which indeed the rudiments of flowers are sometimes found. But in 

 most of the many cases I have investigated - I could find no trace of the 

 formation of flowers on the bristles, and this circumstance shows that a 

 sharp distinction between abortion and the suppression of the development 

 of shoots cannot be drawn. If we find upon a bristle of Setaria in one 

 case an almost complete spikelet, in another only a trace of the glumes, 

 and in a third no rudiment at all of a spikelet, these three stages must 

 be regarded as only different in degree from one another. Similar 

 examples may be drawn from the formation of leaves. Schmitz states 3 



1 [The term Arrest is here used as the equivalent of the German ' Verkiimmerung ' in its widest 

 application and as including Abortion or partial arrest (' Verkiimmerung ' in its narrower sense, or 

 ' Abortus'), and Suppression or complete arrest (' Nichtanlegung'). The latter terms are used in the 

 same sense as they are by Masters in his ' Teratology.'] 



2 Goebel, Beitr. zur Entwicklungsgeschichte einiger Inflorescenzen, in Pringsh. Jahrb. xiv. 



3 Schmitz, Die Blutenentwicklung der Piperaceen, in Hanstein, Botan. Abhandlungen, ii. p. 37. 



