400 



NATURE 



[February 21, 1895 



increase in thickness of the cell-will takes place from without 

 or from within. He decides, chiefly from observations on the 

 cells of ih; testa in seeds, in favour of apposition from the 

 interior, a conclusion which no doubt holds good for the great 

 majority of cases, though the researches of Strasburger have 

 shown that this is not a universal rule. 



Not long afterwards, Prin^sheim made his first investigation 

 of a question which continued to occupy him, at intervals, for 

 more than thirty years, namely, the development of the Sapro- 

 legniacex, a family which lies on the confines of ihe Alga: and 

 the Fungi. At that time the delimitation of genera and species 

 among these plants had fallen into great confusion. Prings- 

 heim's memoir of 1851 bears the title, "On the development 

 of Achlya frolifera," but its suhject was really a SafroUgnia. 

 The zoospores of this family had long been known, and their 

 re<tingspores had already been observed by Schleiden and 

 Nageii. Pringshcim, however, was the first to prove that the 

 two kinds of spore are produced by the same species ; he 

 watched the germinalion of the resling-spores, and saw that 

 they gave rise to the ordinary form of the plant, with its 

 zoospores. At that time he regarded it as an open question 

 whether these plan's are to be classified as .\lgce or Fungi ; they 

 are now assigned to the latler class, but placed near its lower 

 limit, where there has not been much divergence from the 

 original algal stock. 



Six yeais later, Pringsheim discovered the sexual reproduction 

 of the Saprolegniace-Te, an observation of the greatest im- 

 portance, though his account was not free from mistakes, most 

 of which he corrected himself in subsequent memoirs. It is a 

 remarkable fact that in certain species of the family some indi- 

 viduals are entirely without male organs, and produce their 

 otjspores parthenogenctically. In others the antberidia are 

 present, but there is no penetration of the oogonium. The 

 question whether actual fertilisation takes place in aiij' Sapro- 

 legniace.u was the subject of a keen controversy between 

 Pringsheim and De Bary ; the latler, for reasons w hich we have 

 not space to enter into, held that the family, as a whole, is 

 afnganwui, i.e. that the sexual organs, even when present, have 

 lost their function. Pringsheim, on the other hand, maintained 

 that in the forms with fully developed antheridia true fertilisation 

 takes place by means of "spetmamoebx." The question was 

 under discussion up to the year 1883, and cannot even now be 

 regaided as finally settled. Pringsheim's observations of the 

 supposed amccboid male cells are not very convincing, but 

 neither is Ihe negative evidence against the occurrence of 

 fertilisation in the family deci^ive. The higher Fungi are 

 characterised, as most botanists now agree, by a total loss of 

 sexuality ; in Ihe .Saprolegniaceae we can, at any rale, trace ihe 

 beginnings of this retrogressive change. 



It was, however, among the Algae themselves that Prings- 

 heim's chief triumphs were won. His earliest purely Algo- 

 logical paper was on the germinalion of the reslings|iores 

 of S/irn/!jra, in which he first demonslraled ihe interest- 

 ing fact that the embryonic plant presents a distinction of apex 

 and b.ise, which is quite lost in the later stages of its growth. 

 The supposed motile spores of Spirogyra (described in the 

 same paper, 1852) really belonged to a Chylridiaceous 

 paiasite. These inconvenient intruders had already misled 

 Pringsheim in some of his earlier observations on Sapro- 

 legniacex. In Ihe same year, he made out the reproduction in 

 an interesting little Alga {Calastnim) allied to the " Water- 

 nel." 



Pringsheim's really fundamental investigations of the Alga: 

 begin, however, with the year 1855, when he communicated 

 to Ihe lierlin Academy a memoir on Ihe fertilisation and 

 germination of Ihe Algtc, and on the nature of sexuality. The 

 chief point of lhi« paper is the demonstration of Ihe sexual 

 icptoduclion of I'aiiihrria. Vaucher himself had discoveteil 

 the "anthers" at Ihe beginning of ihe century ; it was re- 

 •erved for Pringsheim to find the spermalor.oids, an<l loobserve 

 the act of ferlili-alion. The proof of such an advanced mode 

 of repro<luction in an unicellular (or rather non-cellular) plant, 

 was a startling discovery, and provoked some opposition 

 among those who had not Ihe .»kill \'> repeat Pringsheim's 

 obsetvalions. This paper fuilhtr contains researches on the 

 fertilisation of /iicu!, and on the sexual organs of Flnridca: 

 fwhich of course was only fully understood at a laler time), and 

 preliminary observations on the rcmaikable life-history of 

 CEdn^aniiim and llulboihtilt. 

 Theie latler discoveries were more fully announced in the 



NO. 



I 32 I, VOL. 51] 



next year (1S56), and two years later the whole history was told 

 in a magnificent memoir, published in the first number of the 

 Jahrl>ii(her fiir \Vi:stnschafilichc Bolanik. Tins work is a 

 model for all such investigations, and has left very little for sub- 

 sequent observers to do, as regards these plants, which in some 

 respects present unique peculiarities. The Gidogoniacere 

 possess (in addition to the asexual zoospores) highly difleren- 

 tialed ova and spermatozoids. Many species are moncccious, 

 and a few dia^cious ; most, however, present a remarkable form 

 of dimorphism. Certain cells produce ova ; others (usually in 

 the same plant) give rise to small zoospores, which become 

 free, attach themselves to the oo^onia, or to adjacent cells, 

 and there germinate into dwarf male plants. Each of these 

 dwarfs consists of a single vegetative cell and an antheridium, 

 in which two or more spermatozoids are produced, by which 

 fenilisation is effected. 



Another interesting point discovered by Pringsheim, is the 

 germination of the sexually produced resling-spores, which do 

 not grow directly into new plants, but give rise, each, to four 

 zoospores ; we thus have a case here, either of polyembryony, or 

 of alternation of generation, according to our interpretation of 

 the phenomena. 



Two years later (in i860), Pringsheim published an equally 

 important memoir on another group of freshwater Alg.-e — the 

 Coleocha;tea;. These curious little plants, which grow on the 

 larger water-weeds, go through one ci cle of development in each 

 year. During the earlier part of the season, only asexual in- 

 dividuals, reproduced by zoospores, are formed. At the close 

 of summer the last generation produces sexual organs also. 

 After fertilisation (which is effected by spermatozoids) the 

 oospore passes into its winter rest, and then germinates in the 

 spring, giving rise, not to a normal plant, but to a paren- 

 chymatous, fruit-like body, in e.ach cell of which a zoospore is 

 formed. There is thus an evident analogy with the life history 

 of the Mosses, which Pringsheim at once recognised. Of all 

 Thallophytes, the Coleoch.-eleae show the nearest approach to 

 that form of alternation' of generations which is so general 

 among the Bryophyta. Whether this is anything more than an 

 analogy, is still an open question. 



A controversial .-Vlgoloyical paper of the same date (i860) is 

 of interest, because the author opposes the view (held at one 

 lime by Thuret) that only a dytiamical reaction lakes place be- 

 tween spermatozoid and ovum. Pringsheim showed that the 

 two sexual cells undergo an actual material fusion, a fact which 

 lies at the root of all sound views as to the nature of the sexual 

 process. 



A year later, Pringsheim added to our knowledge of the re- 

 markable course of development of the "Water-net" (Hydr, 

 dictyon), by tracing the history of those minute zoospores which 

 become free from the parent-plant, and give rise to resling- 

 spores. In this case, however, he overlooked the occurrence of 

 conjugation, which, according to a subsequent observer, lakes 

 place between these small motile cells. 



In 1862, Pringsheim turned his attention to marine Alg.x, and 

 published some observations on the Red Seaweeds, in which 

 he rightly described the structure of the sexual organs. Their 

 function, however, was only demonstrated, five years later, by 

 Thuret and Hornet. 



To the year 1863 belongs an important memoir on the 

 Characea-, a group of plants, which, though so popular as lexl- 

 b)ok types, is still a mystery as regards its relationships. 

 Pringsheim's observations were cliiedy in the proembryo and 

 allied structures. He established the existence of a striking 

 correspondence between these organs and the i)rotonema of the 

 Mosses. 



After an interval of six years, he returned, in 1869, to 

 the investigation of the .Mg.x, and made known perhaps the 

 most important of all his Algological discoveries, that of the 

 conjugation of zoospores, a process which he regarded as the 

 primitive form of sexual reproduction in plants. 'This striking 

 iliscovery was first made in /'atiJorhia, one of the Volvocineic, 

 a family whch is often claimed by the zoologists, but which, in 

 its reproiluclive phenomena, bclrays the closest aflinity to 

 undoubted plants. The motile cells, which conjugate in pairs, 

 sonipiimcs difler considerably in size, while in other cases they 

 are almost exactly similar. I'he product of their union is a 

 resting-spore, which on germination ultimately, though not 

 direcily, gives rise to a new colony. Pringsheim observed the 

 details of the fusion of the sexual cells, and found that they 

 first become united by their anterior ciliated ends, which consist 



