CHAPTERS ON EVOLUTION. 



end that of forming a climbing support served by a different 

 means, when we turn to the Smilax (Fig. 65), which in Southern 

 Europe replaces the Bryony of our English hedgerows. The leaves 

 of Smilax are heart-shaped, and when we look at the points at which 

 the leaves spring from the stem, we detect two tendrils (//), which 

 pass to the surrounding plants there to entwine 

 themselves in complex fashion. Now, what are the 

 tendrils of Smilax ? Our knowledge of the leaf and 

 our observation of the position of our tendrils enable 

 us to answer the question. What organs arise from 

 the base of the leaf-stalk? The reply, illustrated by 

 a reference to Fig. 62, is " stipules " (s s) ; and stipules 

 are paired organs. Therefore, we conclude that the 

 tendrils of Smilax are simply altered stipules. The 

 LEAVES OF SMILAX. Yellow Vetch (Fig. 66), which adorns our cornfields, 

 reverses the conditions of Smilax. The stipules (s s) 

 remain in the Vetch to represent the leaves, whilst the leaf-stalk 

 itself and its leaflets become altered as in the Pea, to form tendrils 

 (/ 1) and to enable Lathyrus to indulge its climbing propensities. 

 Thus does a study of tendrils illustrate in apt fashion the bearings 

 of homology. But for this science of likenesses we should not be 

 enabled to unravel some of the complexities which beset the study 



of how a plant climbs ; and we again 

 note how modification and adapta- 

 tion, as distinguished from new crea- 

 tions, form the way of the world of life. 

 No less interesting in certain of its 

 aspects is the study of the " thorns " 

 and " prickles " which " set the rose- 

 bud," or give to the hawthorn its 

 characteristic name and feature. The 

 popular botany of every-day life is 

 content to consider prickles and 

 thorns to represent one and the same 

 kind of structure. But the science of likenesses is careful to ask us 

 to make a very decided distinction between their nature as between 

 the tendrils themselves. Examine the Sloe (Fig. 67, A), for instance, 

 or the Hawthorn, and you will readily determine the nature of the 

 "thorns" which these plants bear. You will note that from the 

 thorns (a a) leaves spring, and in this observation lies the key to the 

 understanding of their relationship with other parts of the plant 

 Leaves are only borne on the stem itself or on the appendages of 

 the stem we familiarly call branches. Therefore the presence of 

 leaves on the thorns, plainly tells us that these appendages of Sloe 

 and Hawthorn are in reality stunted branches. Nor are we left in 

 the slightest doubt as to the nature of these objects ; for many of the 



FIG. 66. YELLOW VETCH. 



