VASCULAR SYSTEM IN ROOTS 385 



one side of the haulm, when it is more brightly illuminated and 

 hence for the time being more actively engaged in transpiration and 

 photosynthesis, to enlist the vascular strands of the other side in its 

 service, as well as its own bundles. In order to test the water- 

 conducting capacity of these vascular cross-connections, the author 

 carried out the following experiment upon Scirpus lacustris. Deep cuts 

 were made in a number of haulms, each incision extending over about 

 one-third of the circumference. The. strips of tissue above the cuts 

 remained perfectly fresh and turgid, and the wounded haulms were 

 in other respects indistinguishable from uninjured specimens, even 

 after the lapse of several weeks. 



In certain Monocotyledons, the course of the vascular strands differs 

 so greatly from the scheme of the Palm-stem, as to necessitate the 

 recognition of distinct types of vascular structure. These aberrant 

 plans of arrangement approximate in many respects to the ordinary 

 Dicotyledonous condition. 



B. COURSE OF THE VASCULAR BUNDLES IN ROOTS. 



An ordinary subterranean root of a Vascular Cryptogam or 

 Phanerogam typically contains an axile vascular strand, from which 

 similar bundles pass into the lateral roots without any further 

 complication. The simplicity of the vascular arrangement is, in the 

 first instance, correlated with the fact, that the medium in which roots 

 grow is more or less constantly moist, so that the cortex is hardly ever 

 exposed to the danger of drying up, even in those parts of the root 

 which have already cast off their absorbing tissue. In these circum- 

 stances peripheral bundles are not needed. The central position of the 

 vascular bundle also accords with the inextensible character of the 

 organ ; in the absence of specialised mechanical elements, the vascular 

 tissue can cope with moderate mechanical requirements, if it is massed 

 around the longitudinal axis of the root. From a physiological point 

 of view, therefore, the central mestome-core of a root is in many 

 respects comparable to the axile strands which so frequently occur in 

 the stems of water-plants. 



Some account has already been given, in an earlier part of the present 

 chapter, of those anomalies in the vascular structure of roots which 

 consist in the appearance of numerous isolated vessels and leptome- 

 strands within a dilated central cylinder. Keinhardt has carried out 

 researches with regard to the relations of the accessory bundles to one 

 another, and to the normal hadrome- and leptome-plates, in such cases. 

 It appears that completely isolated hadrome-strands, extending from the 

 stem to the apex of the root, occur in a number of Palms (Caryota 

 purpurascens, Phoenix daetylifcra, species of Cocos and Chamaedorea, etc.). 



2b 



