TISSUES AND SIMPLE ORGANS. 61 



other plants. These are the so-called " scissiou-phelloids " of v. 

 HoHNEL," which liave the function of bringing about the scaling of 

 the bark. 



The scar-cork or scar-tissue has been mentioned above. When 

 the living cells of various tissues are injured or killed, the neigh- 

 boring cells are sometimes enabled to create a protective covering at 

 once, having therefore the behavior of cork-cambium. It is not 

 within the province of this book to enter into a discussion of prac- 

 tical arboriculture. I cannot, however, omit pointing out the funda- 

 mental principles underlying all those operations which are of such 

 importance in fruit-tree culture, namely grafting. In the various 

 kinds of grafting, such as root-grafting, side-grafting, saddle-graft- 

 ing, bud-grafting, etc., injuries must of necessity occur ; while in all 

 cases an effort is made to induce the separated parts to grow 

 together. One essential to bring about such a union is that cam- 

 hium must he iti contact with cambium. The growing together of 

 separated tissues sometimes takes place during the natural develop- 

 ment of plants ; but caution is necessary in the explanation of such 

 phenomena in order to avoid the mistake of pronouncing tissues as 

 having grown together which were in reality never separated. 

 The phanerogamic parasites form a growth-union with the host 

 plants, while the basal parts of sympetalous (united petals) corollas 

 have never been separate. 



Structural Aids to the Function of Cork-tissue and Cuticula 

 (cuticle). — Trichomatic Organs (trichomes). — In harmony with the 

 subject under discussion the question might arise, Are there still 

 other structures, besides the epidermal system with its cuticular and 

 cork-formations, which serve to protect plants against excessive 

 drying ? As is to be expected, this question is answered in the 

 affirmative. Among other works, the reader interested in this sub- 

 ject will find valuable information in Yolkens' " Flora of the 

 Arabian Desert " (Berlin, 1887). I will touch briefly upon the 

 salient details. 



The limitation of the entire life of desert plants to the most 

 suitable period of the year (period of rainfall), therefore also the 

 hastening of the vegetative period, then the transfer of the time of 



1 Wiener akad. Sitzungsber., LXXVI, 1. Abtlieilung. J. E. Weiss has also 

 written on the same subject (Deukschrift. d. K. Bot. Ges. zu Regeusburg, 1890, 

 VI). 



