428 



KNOWLEDGE. 



November, 1913. 



Chaveaud believes the stem cylinder in the upper hypocotyl 

 of a fairly old seedling to be a true stele, but one belonging to 

 a later phase of evolution than that of the root, and not 

 strictly homologous with it in the sense in which the earliest 

 vascular formations in cotyledon and hypocotyl respectively 

 were homologous with each other. He considers that 

 the successive vascular formations — marked by the 

 appearance of alternate, intermediate, and superposed xylem 

 in turn — represent three successive phases of stelar 

 development : the root stele corresponds with the first 

 of these phases only. This implies the hypothesis that at 

 some past period a group of plants in the direct line of descent 

 of Angiosperms possessed a stele resembling that which is now 

 a mere stage in the life of the individual ; thus the alternate 

 formation found throughout the very young seedling implies 

 an ancestral group with an exarch stele in stem as well as 

 root, and a leaf-trace of corresponding structure. If this view 

 be adopted, the seedling must, during the period when it 

 consists only of cotyledons, hypocotyl, and primary root, with 

 the plumule present as a mere bud, represent a past period in 

 race history when its ancestors possessed an exarch stele in 

 both stem and root, when the stem stele belonged to the stem 

 only and the insertion of leaf-traces hardly modified its 

 structure, and when it entered the root without change, and 

 therefore no transitional region occupied and puzzled the 

 anatomist of the period ! This early stage in the development 

 of the seedling is succeeded by that in which the epicotyl 

 (plumule axis) begins to grow, and as a rule the epicotyl is 

 undoubtedly modern : its vascular skeleton is built up of leaf- 

 traces which are endarch from the first, at the cotyledonary 

 node they are inserted on the vascular cylinder of the 

 hypocotyl which has become endarch at the top. This 

 transition has been effected lower down in the hypocotyl, as 

 described already, by the formation first of intermediate and 

 then of superposed xylem together with the gradual disappear- 

 ance of the original alternate xylem. Hence the cotyledonary 

 node may be considered to mark the interval between two 

 acts in the drama of evolution — an interval the length of which 

 cannot yet be estimated, but is clearly to be reckoned in 

 geological epochs. The race history of the phanerogamic 

 stem-cylinder is at present unknown ; possibly the develop- 

 ment of the hypocotyl may give a clue as suggested by 

 Chaveaud, or Jeffrey may be right in deriving the leaf-traces 

 from a simple tubular stele (siphonostele) which has become 

 more and more broken up by the appearance of foliar gaps. 

 Until this point is cleared up the exact relationship of the 

 vascular cylinder of the stem to that of the root will remain 

 obscure ; as a matter of convenience the stem cylinder will no 

 doubt be called a stele, even should anatomists acknowledge 

 that it cannot be considered as strictly homologous with the 

 stele of the root, but much confusion of thought would be 

 avoided if the two structures were not treated as strictly 

 comparable. 



Apart from the foregoing consideration of modern 

 embryology in relation to a single problem of internal 

 anatomy, namely, the comparison of the vascular system of 

 the stem to that of the root, the evidence of embryology is 

 of great weight in questions of internal morphology and 

 phylogeny. Hanstein's account of the Monocotyledon 

 embyro suggests two distinct problems : (1) whether a 

 terminal member can be considered as a leaf, (2) whether 

 Dicotyledons are derived from a monocotyledonous ancestor, 

 or Monocotyledons from a dicotyledonous form. The most 

 obvious interpretation of Hanstein's observations is that 

 the single cotyledon of Monocotyledons is equivalent to the 

 pair found in Dicotyledons : this would imply that Dicotyle- 

 dons were derived from an ancestor with one cotyledon, 

 apparently terminal, which gave rise to the existing pair by a 

 process of splitting ; but other interpretations are possible, and 

 the terminal hypotheses received a shock when Solms-Laubach 

 discovered that in certain Monocotyledons the single cotyledon 

 is lateral from the first. The comparative antiquity of Mono- 

 cotyledons and Dicotyledons has been one of the first 

 questions raised by the study of seedling anatomy, and it is 

 remarkable that both the hypotheses founded on work of this 



kind assert the greater antiquity of the dicotyledonous form ; 

 but if the cotyledonary member of Monocotyledons is derived 

 from one or both cotyledons of an ancestral pair, it cannot 

 be considered as terminal. Thus the evidence of seedling 

 anatomy bids fair to settle both these problems, and probably 

 others of the same kind. 



Though the progress of botanical embryology has been 

 here treated from the morphological side only, it is clear that 

 every department of botany must deal with the immature 

 plants as well as with the adult form. For instance, the 

 struggle for existence between two species in any particular 

 locality must be profoundly affected by the characters of their 

 seedlings. If one species should gain a decided advantage 

 over the other early in life, the vanquished species may never 

 live to set seed, and may thus disappear from the neighbour- 

 hood in the first generation. This is an extreme case to show 

 the importance of considering seedling structure in problems 

 of ecology and distribution. The internal structure of seed- 

 lings is certainly a department of vegetable anatomy, just as 

 their adaptation to the conditions of life is a department of 

 vegetable physiology. That the connection between embryo- 

 logy and systematic botany must be equally close seems 

 at first sight to be beyond dispute, but the exact nature 

 of that connection is as yet undetermined. Certain features 

 of the embryo are included among the characters used 

 by systematists, but on the whole, the latter have dealt 

 exclusively with the adult plant, the embryo itself having been 

 treated rather as a portion of the seed than as an individual. 

 We need not be surprised if conclusions drawn from the new 

 embryology — that is, the embryology which includes internal 

 characters as well as external — sometimes appear to conflict 

 with the results of systematic botany, and it does not 

 necessarily follow that embryological evidence is of no 

 systematic value. The fault may lie with the embryologists 

 who, being human, do occasionally misinterpret their facts, or 

 possibly the natural system may need some modification in 

 the light of new knowledge. When both explanations have 

 failed to account for the discrepancy in a number of cases, we 

 may be forced to give up looking for phylogenetic results from 

 embryology. 



A summary of various papers read at the Birmingham 

 meeting will be given in these columns next month. 



CHEMISTRY. 



By C. Ainsworth Mitchell, B.A. (Oxon), F.I.C. 



PATHOGENIC ORGANISMS IN RIVER WATER.— 

 Mr. A. C. Houston deals, in the Ninth Research Report, 

 Metropolitan Water Board, with the question of the typhoid 

 bacillus and its distribution. In a former Report, an outline 

 of which was given in these columns, it was shown that under 

 the ordinary conditions of storage and filtration the sewage 

 micro-organisms in a polluted river water are so diluted that 

 a very large volume of the water must be examined in order to 

 detect a single typhoid bacillus. " Where then," as Mr. 

 Houston asks, " is the typhoid bacillus ? " and he supplies the 

 answer in the following words : " The home of the typhoid 

 bacillus is not so much in impure waters, or even in the 

 crude sewage from a large community, as in the ' factories ' 

 of disease, as exemplified by the ' carrier ' case." 



In other words, there may be much less danger in the 

 contamination of water by the sewage of a whole community 

 than in traces of contamination from a single individual who 

 happens to be what is known as a " typhoid carrier." In the 

 case of the ordinary sewage contamination produced by a 

 large town the effect of these unknown typhoid " carriers " is 

 reduced by dilution, so that the water will probably contain 

 only the normal quantity of typhoid bacilli. Incidentally it is 

 pointed out that there is but little risk of typhoid infection 

 from breathing sewer gas, and it is interesting to recall that 

 this confirms the conclusion of a former editor of 

 " Knowledge," Mr. A. C. Ranyard, who, twenty years ago, 

 wrote an article on the subject in this journal. 



