384 



KNOWLEDGE. 



October, 1913. 



referred as " the salt of the subject, preserving it from the 

 worst effects of a purely professional and academic standard," 

 gave an interesting account of the development of botanical 

 embryology since 1870. As might have been expected from 

 the large share which the President herself has taken in the 

 investigation of seedling structure in Angiosperms and its 

 phylogenetic interpretation, the address was limited to 

 Angiosperm embryology, though this is, of course, in itself a 

 sufficiently wide field. As pointed out by Balfour, embryology 

 ought, strictly speaking, to deal with the growth and structure 

 of organisms during their development within the egg- 

 membranes before they are capable of leading an independent, 

 existence, but modern investigators have shown that such a 

 limitation of the science would have a purely artificial 

 character, and the term " embryology " is now employed to 

 cover the anatomy and physiology of the organism during the 

 whole period included between its first coming into being and 

 its attainment of the adult state. The older botanists used 

 the term in the narrower sense, including the study of the 

 embryo-sac and the structures contained in it before the 

 formation of the unfertilised egg-cell as well as the fertilisation 

 of the latter and its subsequent divisions, but they did not 

 proceed beyond the resting stage of the embryo within the 

 ripe seed. Here, as in zoology, this division is arbitrary and 

 inconvenient ; hence in the following remarks embryology is 

 taken to include every stage in the development of the plant 

 from the first division of the fertilised egg-cell to maturity. 



Systematists from Caesalpino onwards have paid much 

 attention to the structure of the seed, and were indeed forced 

 to study the embryo because its characters are often of 

 systematic importance ; for instance, the number of cotyledons 

 is the most constant character separating the two great 

 classes of Angiosperms, while the presence or absence of 

 endosperm in the ripe seed, besides being important 

 systematically, determines the function of the cotyledons 

 after germination, and thus influences their structure pro- 

 foundly. Hence botanists became familiar with the structure 

 of the embryo in the ripe seed before they had traced its 

 origin from the fertilised egg-cell, or followed its development 

 after germination. Since the early history of the embryo was 

 a sealed book to observers without the compound microscope, 

 work on the external morphology of seedlings preceded that 

 on the formation of the embryo. In the school of seedling 

 descriptive work the greatest name is that of Thilo Irmisch 

 (1815-1879), whose work was neglected by the succeeding 

 generation owing to the rapid development of microscopic 

 botany, starting from Hanstein's classic work (1870) on the 

 divisions of the fertilised egg-cell, which laid the foundation of 

 botanical embryology in the narrower sense — the study of the 

 embryo from origin to germination. The period in the plant's 

 history beginning with the first division of the fertilised egg 

 (a natural epoch, since a new generation dates from it) and 

 ending with the formation of the ripe seed (a true physiological 

 epoch, since it corresponds with a complete change in the 

 conditions of life) would seem very well defined ; but 

 experience has shown that here, as in zoology, embryologists 

 lose more than they gain by dividing the subject in this way — 

 one group of investigators beginning their work where the 

 others end theirs — and that this division is neither so simple 

 nor so natural as it appears at first sight. It is not simple 

 because the embryo is not always completely dormant during 

 the interval between the formation of the ripe seed and the 

 first steps in germination. In most Monocotyledons and 

 many Dicotyledons the embryo is an almost undifferentiated 

 mass of meristem when the seed first ripens, and becomes 

 differentiated internally and externally by degrees during the 

 interval before germination : this is often called the matura- 

 tion of the seed, and it is quite distinct from its ripening. 

 Maturation is a process characteristic of the seeds of geophilous 

 plants (plants with bulbs, corms, rhizomes and other perennat- 

 ing underground organs) which commonly lie in the ground 

 for a year at least before germination ; the embryo of such 

 plants is not comparable morphologically with that in the seed 

 of an annual which may have ripened at the same time, since 

 the embryo of the annual has root, stem, and leaves besides its 

 cotyledons, and is ready to germinate immediately on the 



return of spring. Hence the morphologist must continue his 

 study of the geophilous embryo throughout the maturation 

 period if he is to compare it with that of the annual ; 

 even then he will find it less advanced than the annual 

 embryo though both be examined as they break out of 

 the seed, for the geophyte may, perhaps, be four or five years 

 before it flowers, while the annual has to complete its whole 

 life-cycle in a single season. The division of the subject into 

 two parts, the first ending with the embryo in the ripe seed, 

 is also an unnatural one, even if the time of maturation be 

 included in that first period ; for the structure of the embryo 

 cannot be completely understood by reference to its past 

 alone. The observer must expect adaptive characters of 

 three kinds : — (1) those imposed on the embryo in the past 

 by its development within the embryo-sac while it is still 

 parasitic on the parent plant ; (2) certain adaptations to the 

 process of germination itself ; (3) characters which will be 

 useful after germination. Before the utility of these characters 

 can be fully understood the development of the seedling must 

 be followed for some time. In short, the structure of the 

 embryo is dependent upon its future as well as on its past, 

 and a division of the subject which excludes that future is, as 

 Balfour says, purely artificial. 



The work done in recent years on the anatomy of the 

 seedling has therefore not only completed Irmisch's work on 

 its external morphology, but has also thrown light on the 

 problems of early embryology, attacked by Hanstein and his 

 immediate followers. These problems are of two kinds, 

 relating to the internal anatomy or the external morphology 

 of the embryo. Hanstein himself was chiefly interested in 

 the former, and his work disposed once for all of the 

 possibility that the embryo of Angiosperms might possess 

 an apical cell in the earlier stages of its growth as a 

 reminiscence of its cryptogamic ancestry. One general result 

 of work on the embryo since Hanstein's time has been 

 to discredit phylogenetic theories based on its early 

 history ; indeed, it was hardly to be expected that a small 

 mass of meristem, developing within a confined space and 

 feeding parasitically on the tissues of the mother-plant, should 

 preserve ancestral features. Still, Hanstein and his successors 

 did good service in elucidating the growth of the pro-embryo 

 from the fertilised egg-cell ; its division into suspensor and 

 embryo ; the general development of both, and the appearance 

 of external and internal differentiation in the embryo before 

 germination. 



While some of Hanstein's general conclusions as to internal 

 anatomy have become the common property of text-books, for 

 instance, the early differentiation of the dermatogen and its 

 subsequent development into the epidermal system, he was 

 less successful in demonstrating the initial independence of 

 plerome and periblem, and their relation to the vascular 

 system of the mature stem. The early differentiation of 

 plerome and periblem from the inner tissues of the embryonic 

 axis and their continued formation at the growing- points of 

 root and shoot are processes which demand the most careful 

 investigation on account of their bearing on the stelar hypo- 

 thesis. 



(To be continued.) 



CHEMISTRY. 



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



TESTING THE VITALITY OF SEEDS— An 

 ingenious method of ascertaining whether a seed is living 

 has been devised by Mr. S. Tashiro, and an outline of his 

 communication on this subject to the Eighth International 

 Congress of Applied Chemistry is given in the Analyst (1913, 

 XXXVIII, 370). The method depends upon the fact that 

 so long as a seed is alive it liberates carbon dioxide. This 

 gas can be detected and estimated by means of a special 

 apparatus which is so sensitive that it can measure as little 

 as one ten-millionth part of a gramme. It was found with 

 the aid of this apparatus that when a living seed was crushed 



