190 EXTERNAL CONFORMATION OF PLANTS. 



stemmed forms all the larger halues of the leaues 'when first formed turn upqjoards, nvh'ile in 

 the thick-stemmed forms they turn downwards. The want of symmetry of the leaves thus 

 indicates, when the position of the bud is inclined, relationships opposed to the direction 

 of gravity, and when the stem is upright no such relationship. In species with short 

 internodes and thick stems only a few lateral shoots become developed, in those with 

 slender stems a great many, as indeed constantly occurs in other cases (Cactaceae, Palms, 

 Ferns, and to an excessive extent in Isoetes). The lateral arrangement of the lateral 

 shoots exhibits the following relationships to that of the mother-shoot : — in all species 

 the posterior side of the lateral shoot, and hence the larger half of the leaves, faces the 

 mother-shoot ; the principal section of a lateral shoot of slender-stemmed species is 

 therefore at right angles to that of the mother-shoot. In thick-stemmed species, where 

 the axillary shoots are nearer one another in front, the principal section of the lateral 

 shoot makes an acute angle with the mother-shoot in front (and thus in procumbent 

 forms, above). As the development progresses, the branches of slender-stemmed species 

 retain nearly their original position ; in thick-stemmed species where the anterior and 

 posterior sides differ greatly, the lateral shoot twists in such a manner that i's posterior 

 side faces in the same direction as that of the mother-shoot. 



I have no more precise information as to the mode of life of different species of Bego- 

 nia, but suppose that those species in which the anterior and posterior sides are distinctly 

 developed, and which do not cling to the ground, may have the power of climbing, like 

 the ivy, although observations which I have had made for this purpose in the botanical 

 garden at Wiirzburg have not yet led to any satisfactory result ; partly because the 

 plants were already too old, partly because the access of light was possibly too small on 

 the anterior side. The researches detailed above on heliotropism do not negative the 

 hypothesis that with stronger access of light from one side the stems of Begoniae may 

 possibly be negatively heliotropic. It appears moreover from the researches of Martins 

 (' Flora brasihensis,' fasc. XXVII, p. 394) that at least some Begoniae cling to rocks and 

 the stems of trees. (On positive and negative heliotropism see Book III, chap. 3, sect. 8.) 



Sect. 28. Characteristic Forms of Leaves and Shoots. — The peculi- 

 arities of thallomes, leaves, axes, and roots which are common to whole classes, 

 orders, or families (the so-called typical properties), are the subject of special 

 morphology and systematic botany ; on the other hand it is the province of physi- 

 ology to study those relationships of organisation by which the members of the 

 plant become adapted to perform definite functions. There are, however, some 

 peculiarities of growth which recur in different divisions of the vegetable kingdom, 

 or which present themselves in striking contrast to the ordinary phenomena, and 

 are for this reason well adapted to bring into prominence the value of general 

 morphological ideas. Peculiarities of this kind are termed characteristic, and they 

 must be briefly mentioned here, chiefly in order to explain some scientific terms 

 which will be used in Book II. We may limit our remarks to leaves and leaf- 

 forming shoots, since the forms of the thallus will be treated in sufficient detail in 

 the chapter on Thallophytes, and those of roots present only slight characteristic 

 differences, to which reference has already been made ; the characteristic form of 

 hairs has already been alluded to in various ways. 



(i) Forms 0/ Leaves. When fully developed, leaves are usually flatly extended 

 plates of tissue, the extension being generally in directions right and left vertically 

 to the median plane or principal section, so that the surface of the leaf hes trans- 

 versely (at right angles or obliquely) to the longitudinal axis of the stem. This is 

 generally quite true for the base of flat leaves ; but the upper part of the surface 



