INFLUENCE OF EXTERNAL STIMULI. THE MEDIUM 263 



to Buchenau l it is dependent upon habitat. It is suppressed on plants 

 which grow on persistently wet places, but occurs on the other hand 

 frequently on those which grow on warm sandy places. We do not 

 however yet know what part these tubers play in the plant-economy, 

 whether they are intended to store water, or are simply a consequence 

 of an arrest of growth in length. Poa bulbosa appears to behave quite 

 like those species of Juncus. 



D. FORMATION OF THORNS AND PRICKLES. 



Lothelier 2 has recently published a number of statements regarding 

 the influence of air, moisture, and illumination upon the formation of 

 organs in plants provided with thorns and spikes, and if these are 

 correct they have a considerable interest ; but they require confirmation, 

 and I, at least, have not yet succeeded in proving the striking effects 

 enunciated by Lothelier. What he maintains is, that thorns, whether 

 they be branch-thorns, as in Ulex, or leaf-thorns, have the ' tendency ' 

 to assume the form of normal twigs or leaves when the plants are 

 grown in an atmosphere saturated with moisture, in other words, the 

 thorn-formation is suppressed, whilst in Robinia the thorns disappear 

 in these conditions. Plants cultivated in moist air are in their anatomical 

 relationships less differentiated, especially in the matter of their scleren- 

 chymatous tissue, and this is known from other investigations. These 

 anatomical relationships, however, do not come into consideration here. 

 It is of course well known that plants provided with thorns and prickles 

 are specially numerous in dry areas ; the floras of desert and steppe 

 regions furnish abundant examples. This may be a consequence of 

 either a direct adaptation to the dry climate, or the survival of such 

 forms as were protected against animals. On the other hand in the 

 North Arctic region, which is characterized by poverty in plant- 

 eating animals, other xerophilous adaptations occur, rolled-leaves and 

 others, but, so far as I know, never prickles or thorns. We may find 

 naturally different grounds for this want : the small number of arctic 

 plants makes the existence of forms which would produce thorns under 

 the influence of the environment less probable ; the shortness of the 

 period of vegetation would favour such plants as did not make use 

 of their assimilation-capacity for the formation of organs useless in 

 the existing conditions, and so on. Among the inhabitants of the 

 dry regions of the high Andes also we do not find, apart from the 



1 Buchenau, Uber Knollen-und Zwiebelbildung bei den Juncaceen, in Flora, iSyi, p. 75. 



2 Lothelier, Influence de 1'etat hygrome'trique et de 1'e'clairement sur les tiges et les feuilles des 

 plantes a piquants, Lille, 1893; also in Revue gene'rale de Botanique, v. p. 480. 



