GROWTH OF ALG2E AND MYXOMVCETES. 



439 



described, various families are found with very considerable deviations from it. In 

 the lectures on Organography, I have already referred in detail to the genus 

 Laminaria, belonging to the subdivision Phseosporeae. In Laminaria, the inter- 

 calary growing-point of the shoot adds to the length of the shoot-axis downwards, 

 producing each year a new frond, on the contrary, upwards ; the new frond is thus 

 interposed between the old frond and the intercalary growing-point. 



Referring to p. 70 for further details, I may fiow make a few remarks on the Algae 

 in the subdivisions Hydrodictyeae and Volvocineae, where a totally different arrange- 

 ment in space of the embryonic and later stages of growth takes place, though even here 

 the sequence with respect to time is still maintained. One of the simplest examples in 

 this connection is afforded by Pediastrum, 

 a plant common in our waters and the 

 development of which is illustrated in Fig. 

 2^. The mature plant A consists of a 

 flat disc, the cells of which are arranged 

 in concentric circles. In the process of 

 reproduction the contents of the cells break 

 up into a large number of small cells, 

 which, enveloped in a vesicle (-S), escape 

 into the water and there swarm for some 

 time with a trembling movement. The 

 young cells, which taken all together have 

 to be regarded as the embryonic condition 

 of a new plant, then arrange themselves 

 spontaneously in the form of a disc, made 

 up of concentric circles ; and as soon as 

 this has taken place, as in C, they all 

 begin to grow, and the very young plant is 

 now in the stage of elongation. Some 

 little time before the end of this phase, 

 however, the third phase of growth — the 

 completion of development of the cell- 

 contents and cell-walls — commences, until the stage A is again reached. 



Furthest removed from all other modes* of growth is that of the Myxo- 

 mycetes. These consist, in their first period of life — that in which nutrition 

 occurs — of naked motile masses of protoplasm, creeping forth from the nutritive 

 substratum as a so-called plasmodium. Such a plasmodium, according to its 

 specific nature in each case, may assume the most various, and often extraordinarily 

 beautiful forms. Not rarely it developes at last into a stem, situated on the sub- 

 stratum, and passing above into a clavate, spherical, or umbreUa-like expansion, as in 

 Fig. 266. So long as this process of construction continues, the entire plant con- 

 sists of soft protoplasm, and in this condition the Myxomycete may be compared, 

 to a certain extent, with the embryonic condition of a higher plant, although even 

 this comparison fails in some respects. Only when the external form is completed 

 does the outer layer of protoplasm harden to a firm membrane, while its inner 

 portions develope tubular filaments of the most various forms, the so-called capillitium : 



Fig. 266. — Didyiniumfirifioceunt (a Myxomycete) in fruc- 

 tification : only tlie solid network produced from the Plasmo- 

 dium is represented (after Rostatinski). 



