94 COMPENDIUM OF GENERAL BOTANY. 



Now we have the opportunity to offer some suggestions in re- 

 gard to the so-callevi 



(/) " Abnormal Structure of Stems.'''' 



The term ' ' abnormal ' ' is only meant to signify that stems (and 

 ultimately also the roots of the plants concerned) of certain plants 

 as compared with the stems of the majority of species indigenous to 

 our northern hemisphere have peculiar sti'uctural conformations. 

 The complex woody cylinder of Sajnndacem., the peculiarly lobed 

 or fissured stem of Bignoniacece^ belong here. From the fact that 

 we are concerned with a group of plants closely related by certain 

 life-processes, — a hiological group., so to speak, — nearly all of which 

 are climbing or twining plants representing various families, the 

 term "abnormal" is scarcely justifiable. Neither do we consider 

 a climbing plant occurring in a group of non-climbing plants as 

 abnormal. The following marks characterize the abnormal ana- 

 tomical structure : reduced diameter of the stem ; diminution of the 

 loose medullary tissue, especially of the central medullary canal ; 

 centripetal tendency of mechanical elements ; loculose structure of 

 tlie woody portion, due to the presence of the elongated, frequently 

 broad, medullary rays, and especially to a transfer of the leptom^e 

 to the woody hody ; large diameter of vessels and sieve-tubes. 

 There are reasons to suppose that the prevailing tendency of the 

 growth in length of the plants under discussion, and the mechanical 

 adaptations to resist torsions and pulling tensions, are the result of 

 their habits as climbers, though the researches in regard to this sub- 

 ject are not conclusive.^ Other so-called abnormal types, are also 

 very probably phenomena of specific adaptations ; for example, in 

 the autumn Begonias very frequently show the presence of vascular 

 bundles in the fundamental tissue, etc. This subject requires 

 further study, although sufiicient has been said to indicate that 

 the term abnormal as applied to the above instances has the same 

 meaning as it would have if applied to aerial roots, since they also 

 have a special adaptive structure. It is, however, very evident 

 that aerial roots are strictly normal for the life-processes of the 

 plants concerned. 



' Concerning this subject there is a study by Ambronn and myself in Flora, 

 1881. Further investigations by Schenk, Beitrage zur Anatomie der Lianen, 

 Jeua, 1893. Also my own publications in regard to Begoniacece and Oampanulacea, 

 Flora, 1879, and Monatsber. der Berl. Akad., 1881. 



