30 



THE GARDENER'S ASSISTANT. 



these organisms were looked down upon and 

 spoken of as "lower" plants, and, so far as 

 structure and organization go, they really are 

 less highly endowed than are the green or higher 

 plants. But their office in the great scheme of 

 life is by no means unimportant. 



Microbes. — Little as we know of the extremely 

 minute organisms known as microbes, including 

 Bacteria, Bacilli, and other forms, it is sufficient 

 already to enable us to appreciate something 

 of their enormous influence. These plants, so 

 minute as to require the very highest powers of 

 the microscope and the aid of colouring matters 

 to render them visible, are, or many of them 

 are, manufacturing chemists on a large scale. 

 They convert some of the insoluble and in- 

 nutritious particles of the soil into the soluble 

 and the nutritious, and in this way they may 

 and do provide nourishment for other plants 

 which otherwise could not exist on such soils. 

 How they accomplish the feat is not fully 

 known, but, at any rate, we have now learnt 

 to look with much more respect upon these 

 humble organisms than our fathers (if they saw 

 them at all) were able to do. 



Symbiosis. — A third category, that of messmates 

 or co-operators, has now to be added to the 

 " parasites " and "saprophytes" It is not only 

 minute bacteria that co-operate with the more 

 highly organized plants for the common benefit, 

 but some of the fungi, big enough to be easily 

 seen by the naked eye, and which used to be 

 looked upon as deadly enemies, are now known 

 to exist peaceably on the roots of some of the 

 higher plants, and even to contribute to their 

 requirements, while themselves deriving benefit 

 from the association. This association for 

 purposes of mutual accommodation is called 

 Symbiosis; and although it may seem outside 

 the range of practical gardening, we mention 

 it here, as it is by no means unlikely that the 

 want of success with certain plants of which 

 gardeners complain may, as suggested by M. 

 Naudin, eventually be found to be due to the 

 absence of some particular bacterium or fungus 

 necessary to the welfare of the plant (fig. 25). 



On the other hand, some of these bacteria are 

 indeed ministers of decay and death. They are 

 always associated with putrescence and corrup- 

 tion, and some of them, by reason of their poison- 

 generating properties, produce disease and death 

 in plants and animals, including human beings. 



Mention is made of these things at the outset 

 for the purpose of impressing on the student 

 that, although it is convenient for him, or per- 

 haps inevitable, to study one plant, or one part 



of a plant, at a time, yet in nature, as a rule, 

 no such isolation occurs. One part influences 

 another, one plant competes with another, some 

 are allies and confederates, others are antagonists 

 or enemies, all are subject to varying effects of 

 climate and soil; and thus the life -history of 

 even the humblest plant is complex to a degree 

 that the student who confines himself to reading 

 about it alone cannot adequately appreciate. 



Chlorophyll - producing or green plants fall 

 into two or more series ; the simplest are pure 

 thallophytes; such are the Algse, which have 



w 



Fig. 25.— Illustrations of Symbiosis. 



1. Roots of the White Poplar with Mycelial covering. 2. Tip of a Root 

 of the Beech with closely adherent Mycelial covering; X 100 (after 

 Frank). In these cases the Mycelial threads on the roots are of 

 fungous origin, deriving nourishment from the roots on which they 

 grow, but at the same time supplying food-material to the roots. 



a "thallus" only. More highly organized are 

 the Mosses, which have distinct traces of stem, 

 leaf, and root as we recognize them in higher 

 plants, and which also present a slight tendency 

 to form vessels as well as cells. Higher still in 

 degree of organization are the Ferns, in which 

 root, stem, and leaf are even more decidedly 

 marked, and in which the woody and vascular 

 tissues are well marked. All these plants increase 

 by division — one cell divides into two, those 

 into others, and so on. In addition to this 

 process of increase, all, or the vast majority of 

 plants, reproduce themselves by the formation 

 of an entirely new plant, which is the result of 

 the union and interaction of two separate cells, 

 the one male, the other female. In the more 

 highly organized of the green plants the male 

 cells, microspores or pollen- grains, are formed 

 within cases called anthers, which form part of 

 the stamens; while the female cell is formed 

 within the ovule, a little body contained within 

 the ovary or immature " fruit ". These matters 

 will require further explanation later on. At 

 present it is sufficient to emphasize the fact that 

 all complete plants pass through an asexual stage, 



