250 C. E. Beecher — Origin and Significance of Spines. 



I. In response to stimuli from the environment acting on most 

 exposed parts. (A,.) 



The action of external stimuli falling on the most exposed 

 parts of organisms is probably one of the most fundamental 

 and fertile causes of spine production, since the relation 

 between cause and effect is more direct and apparent here than 

 by other modes of origin. In a general way, it comprehends 

 all the remaining causes coming under the head of external 

 stimuli, but for present purposes, it will be restricted by the 

 elimination of secondary conditions, such as the indirect pro- 

 duction of spines through differentiation of previous structures, 

 and the action of external forces of selection. 



The ruling forces in plants being so largely vegetative, or 

 those of growth, and the cause of variation being principally 

 physico-chemical and not molar, most of the modifications to 

 produce spines will fall under other categories of origin (B, D) 

 than the one now under discussion. 



In the free swimming forms, however, as the Desmids and 

 Diatoms, the external relations are found to be very much 

 like those of animals. The f rustrule of the Diatom, Attheya 

 decora* 1 is quadrate in outline, and from the angles there 

 extend sharp spinous processes, as represented in figure 32. 

 The frustrule of the Desmid, Stanrastnim cuspidatum^ is 

 composed of two triangular halves, and the spines project from 

 the vertices of the angles. Other species of Staurastrum, 

 Xanthiditim, (X. armatum Ar ), Arthrodesmus {A. octocor?iis b9 ), 

 etc., show similar spine growth from the most prominent por- 

 tions of the frustrules. It is evident that in forms like these 

 having angular outlines, any growth produced by external 

 stimuli will naturally be greatest at the points of these angles, 

 and in conformity with the previous analyses of these factors, 

 a spiniform extension of the tissues would result. 



Among the fresh-water Hhizopoda belonging to the Proto- 

 plasta (= Amozbina), the genus Difflugia affords good examples. 

 D. globidosa* 1 has a nearly spherical shell. In 1). piriformis ," 

 the shell is elongate pear-shaped, and generally round on the 

 summit or fundus, though in rare instances a central spiniform 

 elevation is developed. This tendency becomes fixed in D. 

 acuminata* 1 in which the shell in general form resembles the 

 preceding species, but the fundus is commonly prolonged into 

 a single acuminate process (figure 33), though occasionally two 

 or three spines are found. In D. corona* 1 there is a circlet 

 of spines around the margin of the fundus besides the primary 

 one in the center. Difflugia constricta* 1 is a variable form, 

 with the top of the shell generally smooth, though sometimes 

 it is acuminate, and occasionally it has two or even a cluster of 

 spines (figures 34-36). Euglypha mucronata* 1 has a terminal 

 spine, as in Difflngia acuminata* 1 Placocista spinosa" is a 



