210 PHYSIOLOGICAL BOTANY. PART II. 



cepting in the few species in which no tracheae have been 

 hitherto noticed. They frequently intercommunicate or 

 anastomose by means of lateral branches, and sometimes 

 form a regular network (see art. 27- fig, 15.). They 

 occur in the woody fibre, in the bark, occasionally 

 even in the pith, and very frequently surround the 

 tracheae. They exist in greatest complexity in the 

 root, from whence they proceed in parallel lines up 

 the stem into the leaves and flowers and then return 

 again to the root, the ascending and descending branches 

 anastomosing throughout their course. The movement 

 of the latex can be witnessed only in those parts which 

 happen to be very transparent ; and it has not been 

 actually seen in many plants. The fr'it-ux i-/mttica, 

 Che/iduninui innjiix, and Al'minn pluntiitjn, are the 

 species upon which most of the observations hitherto 

 recorded have been made. Distir&t currents are ob- 

 served traversing the vital vessels, and passing through the 

 lateral connecting tubes or branches into the principal 

 channels. These currents follow no one determinate 

 course, but are very inconstant in their direction some 

 proceeding up and others down, some to the right 

 and others to the left ; the motion occasionally stop- 

 ping suddenly, and theji recommencing. In detached 

 fragments of the plant it will continue from five minutes 

 to half an hour, according to circumstances ; but M. 

 Schultes has been able so to adjust his lens as to witness 

 the flow in the growing plant. The action is sud- 

 denly checked by cold, and again recommences with 

 an elevation of temperature. The effect does not seem 

 to depend upon a contractile power of the tubes, be- 

 cause the latex flows chiefly or entirely from one end 

 of a tube even when it has an orifice open at both 

 extremities. The appearance is very similar to the 

 circulation of the blood in the foetus contained in a 

 bird's egg before the heart is formed ; but is more es- 

 pecially analogous to the circulation of some of the 

 lowest tribes of animals, as in the Diplozoon paradoxum, 

 which may be divided into two parts and the blood 



