382 



BOTANY 



PART II 



Fiii. 308. — Achlya polyandra. Tlie fertilisation of 

 two egg-cells, o, of an oogonium by two tubes 

 from the antheridium a ; ck, nucleus of the egg- 

 cell ; sk, sperm-nucleus ; in 02 the section has 

 not passed through the egg-nucleus. (After 

 Tkow.) 



into the sporangium. Olpidititn Brassicac (Fig. 309), which is parasitic in tlie 

 tissues of the base of the stem of young cabbage plants and brings about their 

 death, may be taken as an example. The sporangia in this case have a long neck 

 from which the swarm-spores escape. Thick-walled sporangia which undergo a 

 resting period are also frequently found in the Chytridiaceae. 



Sexual reproduction is only found 

 in some of the genera, in the form of 

 fusion of an antheridium with an 

 oogonium and the production of an 

 oospore. The systematic position of 

 the Chytridiaceae among the Phycomy- 

 cetes is still doubtful ; the simplest 

 forms appear to have originated in 

 part from Flagellatae and partly from 

 Protococcaceae. 



4. The Peronosporeae (''^) are 

 pai'asitic Fungi whose mycelium pene- 

 trates the tissues of the higher plants, 

 and is frequently the cause of death. 

 In damp climates certain species 

 occasion epidemic diseases in culti- 

 vated plants. Thus, the mycelium 

 of PhytopMhora infestans, the fungus which causes the Potato disease, lives in the 

 intercellular spaces of the leaves and tubers of the Potato plant, and by penetrating 

 the cells with its short haustoria leads to the discoloration and death of the 

 foliage and tubers. Sexual reproductive organs have not as yet been observed in 

 this species. Asexual, oval sporangia are formed on long branching sporangio- 

 phores which grow out of 

 the stomata, particularly from 

 those on the itnder side of 

 the leaves (Fig. 310), and 

 appear to the naked eye as a 

 white mould. The sporangia, 

 at first terminal, are cut off 

 by transverse ■walls from the 

 ends of the branches of the 

 sporangiophore, by the sub- 

 sequent growth of which they 

 become pushed to one side, 

 and so appear to be inserted 

 laterally. Before any divi- 

 sion of their contents has 

 taken place, the s]iorangia 

 (B) fall off and are dis- 

 seminatcii by the wind ; in 

 this way the epidemic becomes widespread. The development of swarm-spores 

 in sporangia is effected only in water, and is consequently possible only in wet 

 weather. In this process the contents of the sporangium divide into several 

 biciliate swarm-spores (C, D). Each of these spores after escaping from the 

 sporangium gives rise to a myccilium, which penetrates the tissues of a leaf. 

 Tiie sporangium may also germinate directly without undergoing division and 



Fi<:. 30Q. —Olindivm Brassirne. A, Three zoosporangia, the 

 contents of one of which has escaped (x KiO). /;, Zoo- 

 spores (X 520). C, Resting sporangia (x 520). (After 



WORONIN.) 



