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单子叶植物的构造与双子叶植物叶的构造相比有什么不同？

答案

参考答案：

①表皮细胞的外壁除角质化外，还充满硅质，使叶的表面形成许多粗糙的小突起，所以叶较坚利.

②上表皮有许多扇形的薄壁细胞，称为运动细胞.

③它的保卫细胞呈哑铃形，其两侧有副卫细胞.

④叶肉细胞中没有栅栏组织和海绵组织之分.

⑤叶脉为平行脉，维管内无形成层.

选择题
下列说法中正确的是（  ）
A. 负数没有平方根
B. 无理数就是带根号的数
C. 有理数和数轴上的点一一对应
D. 两个无理数之和一定是无理数
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完形填空

A professor of environmental health and his students began studying the idea of rooftop gardening for cities. “I think vertical farming is an idea that can work in a big way,” said Despommier.

Why would we want to build skyscrapers filled with vegetables when we’ve been farming on the ground for 10,000 years? Because as the world’s population grows—from 7 billion now to as much as 9 billion by 2050—we could run out of productive soil and water. Most of the population growth will occur in cities that can’t easily feed themselves. And the fact is that modern agriculture as well as everything associated with it – chemical fertilizers and transportation full of carbon—is a significant contributor to climate change, and suddenly vertical farming doesn’t seem so magic in the sky.

“Vertical farming could allow food to be grown locally and sustainably,” says Glen Kertz, CEO of Valcent. The result saves space — vital in urban areas — and allows farmers to irrigate and fertilize with far less waste.

At Valcent’s El Paso lab, crops grow in rows on conveyor belts. Moving them gives the plants the precise amount of light and nutrients needed, letting them grow 15 times as much vegetable as on a normal farm with 5% of the water that traditional agriculture does.

Despommier plans to build a 30-story, city-block-size vertical farm that would have transparent walls to maximize sunlight and would produce enough food for 50, 000 people. But it would cost hundreds of millions to build a full-scale skyscraper farm. That’s the main drawback: construction and energy costs would probably make vertically raised food more costly than traditional crops.

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