638 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



characters of a young vegetative cell, the original zygote- wall remaining on the 

 surface of the leaf as a mere cellulose knob. 



The generations rapidly succeed one another during the summer months, the 

 last-formed cells of the season becoming packed with starch grains and passing the 

 winter in this state. These resting cells can withstand desiccation, in case the pond 

 in which the duckweed lives becomes dried up. 



Phyllobium dimorphum forms large immotile cells between the tracheids of 

 the vascular bundles in the leaves of the creeping Moneywort {Lysimachia 

 nummularia). This plant lives in damp woods and other shady places. The Rhine 

 plain in the neighbourhood of Strasburg, where Phyllohiwm was first found in the 

 leaves of the Moneywort, is usually flooded during the month of June, partly by 

 the rising of the river, and partly by the thunderstorms which usually occur about 

 that time of the year. The Phyllobiwm-ceWa take this opportunity to form their 

 gametes, which are of two distinct sizes, each cell producing gametes of one size 

 only. After the escape of the gametes into the surrounding water conjugation 

 occurs. The zygozoospores produced have only two flagella, the body and flagella 

 of each microgamete being completely lost in the megagamete, just as the body of 

 a spermatozoid is completely lost in the substance of the egg. After coming to rest 

 on the surface of a Lysimachia leaf, and acquiring cell-membranes, the zygotes put 

 out delicate tubes which enter the stomata of the leaf. If a leaf is infected by a 

 few zygotes only, the tubes formed reach the vascular bundles, and forcing their 

 way between the elements of the wood, grow forward in the bundles, branching 

 when they branch, and attaining to a considerable length. Eventually, towards the 

 end of the summer, the protoplasmic contents of each tube becoming concentrated 

 in one spot, this part of the tube swells and is cut off from the remainder by the 

 formation of transverse partitions. The swollen part of the tube thus forms a large 

 cell which rests during the winter, and in the next summer will produce gametes. 

 If, on the other hand, the leaf is infected by a large number of zygotes, most of the 

 tubes never get any further than the intercellular spaces immediately under the 

 stomata. In this position they form small resting cells in large numbers. These 

 eventually form zoospores, which apparently behave, on germination, just like the 

 zygozoospores. The dimorphism of the resting cells of Phyllobium thus depends 

 directly on the amount of space at the disposal of the germ tubes. This conclusion 

 can be confirmed by cultivating the germ tubes apart from the leaves of the host. 



The purpose of the germ tubes of Chlorochytrium, Phyllobium,, and their allies 

 in penetrating the leaves of their hosts, seems to be simply that they may gain the 

 advantage of a quiet protected place for their development. Just in the same way 

 Diatoms and other unicellular forms often live comfortably in the empty cells of 

 Algae, the intercellular spaces of the Bog-moss (Sphagnum), and similar situations. 

 Only in the case of these Endosph^reee the association of the Alga with its habitat 

 is invariable and adaptive, not merely casual and unrelated. But the Endosphserese 

 are not parasites in any sense. They take no food from their " hosts " nor do they 

 exercise any appreciable influence on the latter. This is sufiiciently proved by the 



